grep Pocket Reference

John Bambenek

Agnieszka Klus

Editor

Andy Oram

Copyright © 2009 John Bambenek and Agnieszka Klus

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Chapter 1. grep Pocket Reference

Introduction

Chances are that if you’ve worked for any length of time on a Linux system, either as a system administrator or as a developer, you’ve used the grep command. The tool is installed by default on almost every installation of Linux, BSD, and Unix, regardless of distribution, and is even available for Windows (with wingrep or via Cygwin).

GNU and the Free Software Foundation distribute grep as part of their suite of open source tools. Other versions of grep are distributed for other operating systems, but this book focuses primarily on the GNU version, as it is the most prevalent at this point.

The grep command lets the user find text in a given file or output quickly and easily. By giving grep a string to search for, it will print out only lines that contain that string and can print the corresponding line numbers for that text. The “simple” use of the command is well-known, but there are a variety of more advanced uses that make grep a powerful search tool.

The purpose of this book is to pack all the information an administrator or developer could ever want into a small guide that can be carried around. Although the “simple” uses of grep do not require much education, the advanced applications and the use of regular expressions can become quite complicated. The name of the tool is actually an acronym for “Global Regular-Expression Print,” which gives an indication of its purpose.

GNU grep is actually a combination of four different tools, each with its unique style of finding text: basic regular expressions, extended regular expressions, fixed strings, and Perl-style regular expression. There are other implementations of grep-like programs such as agrep, zipgrep, and “grep-like” functions in .NET, PHP, and SQL. This guide will describe the particular options and strengths of each style.

The official website for grep is http://www.gnu.org/software/grep/. It contains information about the project and some brief documentation. The source code for grep is only 712 KB, and the current version at the time of this writing is 2.5.3. This pocket reference is current to that version, but the information will be generally valid for earlier and later versions.

As an important note, the current version of grep that ships with Mac OS X 10.5.5 is 2.5.1; however, most of the options in this book will still work for that version. There are other “grep” programs as well, in addition to the one from GNU, and these are typically the ones installed by default under HP-UX, AIX, and older versions of Solaris. For the most part, the regular expression syntax is very similar between these versions, but the options differ. This book deals exclusively with the GNU version because it is more robust and powerful than other versions.

Conventions Used in This Book

The following typographical conventions are used in this book:

Italic

Indicates commands, new terms, URLs, email addresses, filenames, file extensions, pathnames, directories, and Unix utilities.

Constant width

Indicates options, switches, variables, attributes, keys, functions, types, classes, namespaces, methods, modules, properties, parameters, values, objects, events, event handlers, XML tags, HTML tags, macros, the contents of files, or the output from commands.

Constant width italic

Shows text that should be replaced with user-supplied values.

Using Code Examples

This book is here to help you get your job done. In general, you may use the code in this book in your programs and documentation. You do not need to contact us for permission unless you’re reproducing a significant portion of the code. For example, writing a program that uses several chunks of code from this book does not require permission. Selling or distributing a CD-ROM of examples from O’Reilly books does require permission. Answering a question by citing this book and quoting example code does not require permission. Incorporating a significant amount of example code from this book into your product’s documentation does require permission.

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Acknowledgments

From John Bambenek

I would like to thank Isabel Kunkle and the rest of the O’Reilly team behind the editing and production of this book. My wife and son deserve thanks for their support and love as I completed this project. My coauthor, Agnieszka, has been invaluable in making an onerous task of writing a book more manageable; she contributed greatly to this project. Brian Krebs of The Washington Post deserves credit for the idea of writing this book. My time at the Internet Storm Center has let me work with some of the best in the information security industry, and their feedback has been extremely helpful during the technical review process. A particular note of thanks goes out to Charles Hamby, Mark Hofman, and Donald Smith. And last, Merry Anne’s Diner in downtown Champaign, Illinois deserves thanks for letting me show up for hours in the middle of the night to take up one of their tables as I wrote this.

From Agnieszka Klus

First, I want to thank my coauthor, John Bambenek, for the opportunity to work on this book. It certainly has been a literary adventure for me. It has opened windows of opportunity and given me a chance to peek into a world I would otherwise have not been able to. I also would like to thank my family and friends for their support and patience.

Conceptual Overview

The grep command provides a variety of ways to find strings of text in a file or stream of output. For example, it is possible to find every instance of a specified word or string in a file. This could be useful for grabbing particular log entries out of voluminous system logs, as one example. It is possible to search for certain patterns in files, such as the typical pattern of a credit card number. This flexibility makes grep a powerful tool for finding the presence (or absence) of information in files. There are two ways to provide input to grep, each with its own particular uses.

First, grep can be used to search a given file or files on a system. For instance, files on a disk can be searched for the presence (or absence) of specific content. grep also can be used to send output from another command that grep will then search for the desired content. For instance, grep could be used to pick out important information from a command that otherwise produces an excessive amount of output.

While searching text files, grep could be employed to search for a particular string throughout all files in an entire filesystem. For instance, Social Security numbers follow a known pattern, so it is possible to search every text file on a system to find occurrences of these numbers in its files (e.g., for academic environments in order to comply with federal privacy laws). The default behavior is to return the filename and the line of text that contains the string, but it is possible to include line numbers as well.

Additionally, grep can examine command output to look for occurrences of a string. For instance, a system administrator may run a script to update software on a system that has a large amount of “debugging” information and may only care to see error messages. In this case, the grep command could search for a string (i.e., “ERROR”) that indicates errors, filtering out information that the administrator does not want to see.

Generally, the grep command is designed to search only text output or text files. The command will let you search binary (or other nontext) files, but the utility is limited in that regard. Tricks for searching binary files for information with grep (i.e., using the strings command) are covered in the last section (Advanced Tips and Tricks with grep).

Although it is usually possible to integrate grep into manipulating text or doing “search and replace” operations, it is not the most efficient way to get the job done. Instead, the sed and awk programs are more useful for these kinds of functions.

There are two basic ways to search with grep: searching for fixed strings and searching for patterns of text. Searching for fixed strings is pretty straightforward. Pattern searching, however, can get complicated very quickly, depending on how variable that desired pattern is. To search for text with variable content, use regular expressions.

Introduction to Regular Expressions

Regular expressions, the source of the letters “re” in “grep,” are the foundation for creating a powerful and flexible text-processing tool. Expressions can add, delete, segregate, and generally manipulate all kinds of text and data. They are simple statements that enhance a user’s ability to process files, especially when combined with other commands. If applied properly, regular expressions can significantly simplify a tall task.

Many different commands in the Unix/Linux world use some form of regular expressions in addition to some programming languages. For instance, the sed and awk commands use regular expressions not only to find information, but also to manipulate it.

There are actually many different varieties of regular expressions. For instance, Java and Perl both have their own syntax for regular expressions. Some applications have their own versions of regular expressions, such as Sendmail and Oracle. GNU grep uses the GNU version of regular expressions, which is very similar (but not identical) to POSIX regular expressions.

In fact, most of the varieties of regular expressions are very similar, but they do have key differences. For instance, some of the escapes, metacharacters, or special operators will behave differently depending on which type of regular expressions you are using. The subtle differences between the varieties can lead to drastically different results when using the same expression under different regular expression types. This book will only touch on the regular expressions that are used by grep and Perl-style grep (grep -P).

Usually, regular expressions are included in the grep command in the following format:

grep [options] [regexp] [filename]

Regular expressions are comprised of two types of characters: normal text characters, called literals, and special characters, such as the asterisk (*), called metacharacters. An escape sequence allows you to use metacharacters as literals or to identify special characters or conditions (such as word boundaries or “tab characters”). The desired string that someone hopes to find is a target string. A regular expression is the particular search pattern that is entered to find a particular target string. It may be the same as the target string, or it may include some of the regular expression functionality discussed next.

Quotation Marks and Regular Expressions

It is customary to place the regular expression (or regxp) inside single quotation marks (the symbol on the keyboard underneath the double quote, not underneath the tilde [~] key). There are a few reasons for this. The first is that normally Unix shells interpret the space as an end of argument and the start of a new one. In the format just shown, you see the syntax of the grep command where a space separates the regexp from the filename. What if the string you wish to search for has a “space” character? The quotes tell grep (or another Unix command) where the argument starts and stops when spaces or other special characters are involved.

The other reason is that various types of quotes can signify different things with shell commands such as grep. For instance, using the single quote underneath the tilde key (also called the backtick) tells the shell to execute everything inside those quotes as a command and then use that as the string. For instance:

grep `whoami` filename

would run the whoami command (which returns the username that is running the shell on Unix systems) and then use that string to search. For instance, if I were logged in with username “bambenek”, grep would search filename for the use of “bambenek”.

Double quotes, however, work the same as the single quotes, but with one important difference. With double quotes, it becomes possible to use environment variables as part of a search pattern:

grep "$HOME" filename

The environment variable HOME is normally the absolute path of the logged-in user’s home directory. The grep command just shown would determine the meaning of the variable HOME and then search on that string. If you place $HOME in single quotes, it would not recognize it as an environment variable.

It is important to craft the regular expression with the right type of quotation marks because different types can yield wildly different results. Beginning and ending quotes must be the same or an error will be generated, letting you know that your syntax is incorrect. Note that it is possible to combine the use of different quotation marks to combine functionality. This will be discussed later in the section Advanced Tips and Tricks with grep.

Metacharacters

In addition to quotation marks, the position and combination of other special characters produce different effects on the regular expression. For example, the following command searches the file name.list for the letter ‘e’ followed by ‘a’:

grep -e 'e[a]' name.list

But by simply adding the caret symbol, ^, you change the entire meaning of the expression. Now you are searching for the ‘e’ followed by anything that is not the letter ‘a’:

grep -e 'e[^a]' name.list

Since metacharacters help define the manipulation, it is important to be familiar with them. Table 1-1 has a list of regularly used special characters and their meanings.

Table 1-1. Regular expression metacharacters[1]

Metacharacter

Name

Matches

Items to match a single character

.

Dot

Any one character

[...]

Character class

Any character listed in brackets

[^...]

Negated character class

Any character not listed in brackets

\char

Escape character

The character after the slash literally; used when you want to search for a “special” character, such as “$” (i.e., use “\$”)

Items that match a position

^

Caret

Start of a line

$

Dollar sign

End of a line

\<

Backslash less-than

Start of a word

\>

Backslash greater-than

End of a word

The quantifiers

?

Question mark

Optional; considered a quantifier

*

Asterisk

Any number (including zero); sometimes used as general wildcard

+

Plus

One or more of the preceding expression

{N}

Match exactly

Match exactly N times

{N,}

Match at least

Match at least N times

{min,max}

Specified range

Match between min and max times

Other

|

Alternation

Matches either expression given

-

Dash

Indicates a range

(...)

Parentheses

Used to limit scope of alternation

\1, \2, ...

Backreference

Matches text previously matched within parentheses (e.g., first set, second set, etc.)

\b

Word boundary

Batches characters that typically mark the end of a word (e.g., space, period, etc.)

\B

Backslash

This is an alternative to using “\\” to match a backslash, used for readability

\w

Word character

This is used to match any “word” character (i.e., any letter, number, and the underscore character)

\W

Non-word character

This matches any character that isn’t used in words (i.e., not a letter, number, or underscore)

\`

Start of buffer

Matches the start of a buffer sent to grep

\'

End of buffer

Matches the end of a buffer sent to grep

[1] From Jeffrey E.F. Friedl’s Mastering Regular Expressions (O’Reilly), with some additions

The table references something known as the escape character. There are times when you will be required to search for a literal character that is usually used as a metacharacter. For example, suppose you are looking for amounts that contain the dollar sign within price.list:

grep '[1-9]$' price.list

As a result, the search will try to match the numbers at the end of the line. This is certainly something you do not want. By using the escape character, annotated by the backslash (\), you avoid such confusion:

grep '[1-9]\$' price.list

The metacharacter $ becomes a literal, and therefore is searched in price.list as a string.

For instance, take a text file (price.list) that has the following content:

123
123$

Using the two commands just shown yields the following results:

$ grep '[1-9]\$' price.list
123$
$ grep '[1-9]$' price.list
123

In the first example, the command looked for the actual dollar-sign character. In the second example, the dollar sign had its special metacharacter’s meaning and matched the end of line, and so would match only those lines that ended in a number. The meaning of these special characters needs to be kept in mind because they can make a significant difference in how a search is processed.

Here is a brief rundown of the regular expression metacharacters, along with some examples to make it clear how they are used:

. (any single character)

The “dot” character is one of the few types of wildcards available in regular expressions. This particular wildcard will match any single character. This is useful if a user wishes to craft a search pattern with some characters in the middle of it that are not known to the user. For instance, the following grep pattern would match “red”, “rod”, “red”, “rzd”, and so on:

'r.d'

This “dot” character can be used repeatedly at whatever interval is necessary to find the desired content.

[...] (character class)

The “character class” tool is one of the more flexible tools, and it comes up again and again when using regular expressions. There are two basic ways to use character classes: to specify a range and to specify a list of characters. An important point is that a character class will match only one character:

'[a-f]'
'[aeiou]'

The first pattern will look for any letter between “a” and “f”. Ranges can be uppercase letters, lowercase letters, or numbers. A combination of ranges can also be used, for instance, [a-fA-F0-5]. The second example will search for any of the given characters, in this case vowels. A character class can also include a list of special characters, but they can’t be used as a range.

[^...] (negation)

The “negation” character class allows a user to search for anything but a specific character or set of characters. For instance, a user who doesn’t like even numbers could use the following search pattern:

'..[^24680]'

This will look for any three-character pattern that does not end in an even number. Any list or range of characters can be placed inside a negated character class.

\ (escape)

The “escape” is one of the metacharacters that can have multiple meanings depending on how it is used. When placed before another metacharacter, it signifies to treat that character as the literal symbol instead of its special meaning. (It also can be used in combination with other characters, such as b or ', to convey a special meaning. Those specific combinations are covered later.) Take the following two examples:

'.'
'\.'

The first example would match any single character and would return every piece of text in a file. The second example would only match the actual “period” character. The escape tells the regular expression to ignore the metacharacter’s special meaning and process it normally.

^ (start of line)

When a carat is used outside of a character class, it no longer means negation; instead, it means the beginning of a line. If used by itself, it will match every single line on the screen because each line has a beginning. More useful is when a user wishes to match lines of text that begin with a certain pattern:

'^red'

This pattern would match all lines that begin with “red”, not just the ones that contain the word “red”. This is useful for structured communication or programming languages, for example, where lines may begin with specific strings that contain important information (such as #DEFINE in C). However, the meaning is lost if it is not at the beginning of a line.

$ (end of line)

As discussed earlier, the dollar sign character matches the end of a line. Used alone, it will match every line in a stream except the final line, which is terminated by an “end of file” character instead of an “end of line” character. This is useful for finding strings that have a desired meaning at the end of a line. For instance:

'-$'

would find all lines whose last character is a dash, as is typical for words that are hyphenated when they are too long to fit on one line. This expression would find only those lines with hyphenated words split between lines.

\< (start of word)

If a user wished to craft a search pattern that matches based on the start of a word and the pattern was likely to recur inside a word (but not at the beginning), this particular escape could be used. For instance, take the following example:

'\<un'

This pattern would match words starting with the prefix “un”, such as “unimaginable,” “undetected,” or “undervalued.” It would not match words such as “funding,” “blunder,” or “sun.” It detects the beginning of a word by looking for a space or another “separation” that indicates the beginning of a new word (a period, comma, etc.).

\> (end of word)

Similar to the previous escape, this one will match at the end of a word. After the characters, it looks for a “separation” character that indicates the end of a word (a space, tab, period, comma, etc.). For example:

'ing\>'

would match words that end in “ing” (e.g., “spring”), not words that simply contain “ing” (e.g., “kingdom”).

* (general wildcard)

The asterisk is probably by far the most-used metacharacter. It is a general wildcard classed as a quantifier that is specifically used for repetitious patterns. For some metacharacters, you can assign minimum and maximum boundaries that manipulate the quantity outputted from the pattern, but the asterisk does not place any limits or boundaries. There are no limits to how many spaces there can be before or after the character. Suppose a user wants to know whether a particular installer’s different formats are described in a file. The results of this simple command:

'install.*file'

the results should output all the lines that contain “install” (with any amount of text in between) and then “file”. It is necessary to use the period character; otherwise, it will match only “installfile” instead of iterations of “install” and “file” with characters in between.

- (range)

When used inside a bracketed character class, the dash character specifies a range of values instead of a raw list of values. When the dash is used outside of a bracketed character class, it is interpreted as the literal dash character, without its special value.

'[0-5]'

\# (backreferences)

Backreferences allow you to reuse a previously matched pattern to determine future matches. The format for a backreference is \ followed by the pattern number in the sequence (from left to right) that is being referenced. Backreferences are covered in more detail in the section Advanced Tips and Tricks with grep.

\b (word boundary)

The \b escape refers to any character that indicates a word has started or ended (similar to \> and \<, discussed earlier). In this case, it doesn’t matter whether it is the beginning or end of the word; it simply looks for punctuation or spacing. This is particularly useful when you are searching for a string that can be a standalone word or a set of characters within another, unrelated word:

'\bheart\b'

This would match the exact word “heart” and nothing more (not “disheartening”, not “hearts”, etc.). If you are searching for a particular word, numerical value, or string and do not want to match when those words or values are part of another value, it is necessary to use either \b, \>, or \<.

\B (backslash)

The \B escape is a peculiar case because it isn’t an escape itself, but rather an alias for another one. In this case, \B is identical to \\, namely, to interpret the slash character literally in a search pattern instead of with its special meaning. The purpose of this alias is to make a search pattern a little more readable and to avoid double-slashes, which could have ambiguous meaning in complicated expressions.

'c:\Bwindows'

This example would search for the string “c:\windows”.

\w and \W (word or non-word characters)

The \w and \W escapes go hand in hand because their meanings are opposite. \w will match any “word” character and is equivalent to ''[a-zA-Z0-9_]''. The \W escape will match every other character (including non-printable ones) that does not fall into the “word character” category. This can be useful in parsing structured files where text is interposed with special characters (e.g., :, $, %, etc.).

\` (start of buffer)

This escape, like the “start of line” escape, will match the start of a buffer as it is fed to whatever is processing the regular expression. Because grep works with lines, a buffer and a line tend to be synonymous (but not always). This escape is used in the same way as the “start of line” escape discussed earlier.

\' (end of buffer)

This escape is similar to the “end of line” escape, except that it looks for the end of a buffer that is fed to whatever is processing the regular expression. In both cases of start and end of buffer escapes, their usage is extremely rare, and it is easier to simply use start and end of line instead.

The following is a list of metacharacters used in extended regular expressions:

? (optional match)

The use of the question mark has a different meaning than it does in typical filename wildcard usage (GLOB). In GLOB, ? means any single character. In regular expressions, it means that the preceding character (or string if placed after a subpattern) is an “optional” matching pattern. This allows for multiple match conditions with a single regular expression pattern. For instance:

'colors?'

would match both “color” and “colors”. The “s” character is an optional match, so if it is not present, it does not cause a failing condition on the pattern.

+ (repetitive match)

The plus sign indicates that the regular expression is looking for a match of one or more of the previous character (or subpattern). For instance:

'150+'

would match 150 with any number of additional zeroes (e.g., 1500, 15000, 1500000, etc.).

{N} (match exactly N times)

Brackets, when placed after a character, indicate a specific number of repetitions to search for. For instance:

'150{3}\b'

would match 15 followed by 3 zeroes. So 1500 would not match, but 15000 would. Note the use of the \b “word boundary” escape. In this case, if the desired match is precisely “15000” and there is not a check for a word boundary “150000”, “150002345” or “15000asdf” would match also because they all contain the desired search string of “15000”.

{N,} (match at least N times)

Like the previous example, putting a number and a comma after it indicates the regular expression will search for at least N number of repetitions. For instance:

'150{3,}\b'

would match “15” followed by at least three zeroes, and so “15”, “150”, and “1500” would not match. Use the word boundary escape to avoid cases where a precise match of a specific number is desired. (e.g., “1500003456”, “15000asdf”, etc.). The use of \b clarifies the meaning.

{N,M} (match between N and M times)

If you wish to match some numbers between two values of repetitions, it is possible to specify both between the braces separated by a comma. For instance:

'150{2,3}\b'

would match “1500” and “15000” and nothing else.

| (alternation)

The “pipe” character specifies alternation inside a regular expression. Think of it as a way of giving the regular expression a choice of match conditions with a single expression. For example:

'apple|orange|banana|peach'

would match any of the strings given, regardless of whether the others are also within the scope of the search. In this case, if the text includes “apple” or “orange” or “banana” or “peach”, it will match that content.

( ) (subpattern)

The last important feature of extended regular expressions is the ability to create subpatterns. This allows for regular expressions that repeat entire strings, use alternation on entire strings, to have backreferences work, and to make regular expressions more readable:

'(red|blue) plate'
'(150){3}'

The first example will match either “red plate” or “blue plate”. Without the parentheses, the regular expression ''red|blue plate'' would match “red” (note the lack of the word “plate”) or “blue plate”. Parenthetical subpatterns help limit the scope of alternation.

In the second example, the regular expression will match on “150150150”. Without parentheses, it would match “15000”. Parentheses make it possible to match on repetition of entire strings instead of single characters.

Metacharacters generally are universal between the different grep commands, such as egrep, fgrep, and grep -P. However, there are instances in which a character carries a different connotation. Any differences will be discussed within the section pertaining to that command.

POSIX Character Classes

Additionally, regular expressions come with a set of POSIX character definitions that create shortcuts to find certain classes of characters. Table 1-2 shows a list of these shortcuts and what they signify. POSIX is basically a set of standards created by the Institute of Electrical and Electronics Engineers (IEEE) to describe how Unix-style operating systems should behave. It is very old, but much of its content is still used. Among other things, POSIX has definitions on how regular expressions should work with shell utilities such as grep.

Table 1-2. POSIX character definitions

POSIX definition

Contents of character definition

[:alpha:]

Any alphabetical character, regardless of case

[:digit:]

Any numerical character

[:alnum:]

Any alphabetical or numerical character

[:blank:]

Space or tab characters

[:xdigit:]

Hexadecimal characters; any number or A–F or a–f

[:punct:]

Any punctuation symbol

[:print:]

Any printable character (not control characters)

[:space:]

Any whitespace character

[:graph:]

Exclude whitespace characters

[:upper:]

Any uppercase letter

[:lower:]

Any lowercase letter

[:cntrl:]

Control characters

Many of these POSIX definitions are more readable equivalents of character classes. For instance, [:upper:] can be also written as [A-Z] and uses less characters to do so. There aren’t good character class equivalents for some other classes, such as [:cntrl:]. To use these in a regular expression, simply place them the same way you would place a character class. It is important to note that one placement of these POSIX character definitions will match only one single character. To match repetitions of character classes, you would have to repeat the definition. For instance:

'[:digit:]'
'[:digit:][:digit:][:digit:]'
'[:digit:]{3}'

In the first example, any single numerical character will be matched. In the second example, only three-digit numbers (or longer) will be matched. The third example is a cleaner, shorter way of writing the second example. Many regular expressions enthusiasts try to accomplish as much as possible with as few keystrokes as possible. Show them the second example, and they may cringe. The third example is a more efficient way of accomplishing the same thing.

Crafting a Regular Expression

Like algebra, grep has rules of precedence for processing. Repetition is processed before concatenation. Concatenation is processed before alternation. Strings are concatenated by simply being next to each other inside the regular expression—there is no special character to signify concatenation.

For instance, take the following regular expression:

'pat{2}ern|red'

In this example, the repetition is processed first, yielding two “t”s. Then, the strings are concatenated, producing “pattern” on one side of the pipe and “red” on the other. Next, the alternation is processed, creating a regular expression that will search for “pattern” or “red”. However, what if you wanted to search for “patpatern” and “red” or “pattern” or “pattred”?

In this case, just like in algebra, parentheses will “override” the rules of precedence. For example:

2 + 3 / 5
(2 + 3) / 5

These two mathematical equations yield different results because of the parentheses. The concept is the same here:

'(pat){2}ern|red'
'pat{2}(ern|red)'

The first example will concatenate “pat” first and then repeat it twice, yielding “patpatern” and “red” as the search strings. The second example will process the alternation subpattern first, so the regular expression will search for “pattern” and “pattred”. Using parentheses can help you fine-tune your regular expression to match specific content based on how you construct it. Even if the rules of precedence don’t need to be overruled for a particular regular expression, sometimes it makes sense to use parentheses for enhanced readability.

A regular expression can continue as long as the single quote is not closed. For instance:

$ grep 'patt
> ern' filename

Here the single quote was not ended before the user pressed Return right after the second “t” (no space was pressed). The next line shows a > prompt, which indicates it is still waiting for the string to be completed before it processes the command. As long as you keep pressing Return, it will keep giving you the prompt until you either press Ctrl-C to break or close the quote, at which point it will process the command. This allows for long regular expressions to be typed in on the command line (or a shell script) without cramming them all on one line, potentially making them less than readable.

In this case, the regular expression searches for the word “pattern”. The command ignores returns and does not input those into the regular expression itself, so it is possible to hit Enter in the middle of a word and pick up right where you left off. Concern for readability is important because “space” keys aren’t easily visible, which makes this example a great contender for subpatterns, to help make the regular expression more understandable.

It is also possible to use several different groupings of strings with their own quotation marks. For instance:

'patt''ern'

would search for the word “pattern”, just as if it were typed with the expected regular expression of ''pattern''. This example isn’t a very practical one, and there is no compelling reason ever to do that with just text. However, when combining different quotation types, this technique makes it possible to take advantage of each quotation type to produce a regular expression using environment variables and/or output from commands. For example:

$ echo $HOME
/home/bambenek
$ whoami
bambenek

shows that the environment variable $HOME is set to /home/bambenek and that the output of the command whoami is “bambenek”. So, the following regular expression:

'username:'`whoami`' and home directory
is '"$HOME"

would match on the string “username:bambenek and home directory is /home/bambenek” by inserting in the output from the whoami command and the setting for the environment variable $HOME. This is a quick overview of regular expressions and how they can be used. There are entire books devoted to the complexities of regular expressions, but this primer is enough to get you started on what you need to know in order to use the grep command.

grep Basics

There are two ways to employ grep. The first examines files as follows:

grep regexp filename

grep searches for the designated regexp in the given file (filename). The second method of employing grep is when it examines “standard input.” For example:

cat filename | grep regexp

In this case, the cat command will display the contents of a file. The output of this command is “piped” into the grep command, which will then display only those lines that contain the given regexp. The two commands just shown have identical results because the cat command simply passes the file unchanged, but the second form is valuable for “grepping” other commands that alter their input.

When grep is called without a filename argument and without being passed any input, it will let you type in text and will repeat it once it gets a line that contains the regexp. To exit, press Ctrl-D.

At times, the output is remarkably large and hard to scroll through in a terminal. This is usually the case with large files that tend to have repetitious phrases, such as an error log. In these cases, piping the output to the more or less commands will “paginate” it so that only one screen of text is shown at a time:

grep regexp filename | more

Another option to make the output easier to look at is to redirect the results into a new file and then open the output file in a text editor at a later time:

grep regexp filename > newfilename

Also, it may be advantageous to look for lines that contain several patterns instead of just one. In the following example, the text file editinginfo contains a date, a username, and the file that was edited by that user on the given date. If an administrator was interested in just the files edited by “Smith”, he would type the following:

cat editinginfo | grep Smith

The output would look like:

May 20, 2008        Smith        hi.txt
June 21, 2008       Smith        world.txt
    .
    .

An administrator may wish to match multiple patterns, which can be accomplished by “chaining” grep commands together. We are now familiar with the cat filename | grep regexp command and what it does. By piping the second grep, along with any number of piped grep commands, you create a very refined search:

cat filename | grep regexp | grep regexp2

In this case, the command looks for lines in filename that have both regexp and regexp2. More specifically, grep will search for regexp2 in the results of the grep search for regexp. Using the previous example, if an administrator wanted to see every date that Smith edited any file except hi.txt, he could issue the following command:

cat editinginfo | grep Smith | grep -v hi.txt

The following output would result:

June 21, 2008        Smith    world.txt

It is important to note that “chaining” grep commands is inefficient most of the time. Often, a regular expression can be crafted to combine several conditions into a single search.

For instance, instead of the previous example, which combines three different commands, the same could be accomplished with:

grep Smith | grep -v hi.txt

Using the pipe character will run one command and give the results of that command to the next command in the sequence. In this case, grep searches for lines with “Smith” in them and sends those results to the next grep command, which excludes lines that have “hi.txt”. When a search can be accomplished using fewer commands or with fewer decisions having to be made, the more efficiently it will behave. For small files, performance isn’t an issue, but when searching through gigabyte-sized logfiles, performance can be an important consideration.

There is a case to be made for piping commands when you wish to search through content that is continually streaming. For instance, if you want to monitor a logfile in real-time for specified content, she could use the following command:

tail -f /var/log/messages | grep WARNING

This command would open up the last 10 lines of the /var/log/messages files (usually the main system logfile on a Linux system), but keep the file open and print all content placed into the file as long as it is running (the -f option to tail is often called “follow”). So the command just shown would look for any entry that has the string “WARNING” in it, display it to the console, and disregard all other messages.

As an important note, grep will search through a line and once it sees a newline, it will restart the entire search on the next line. This means that if you are searching for a sentence with grep, there is a very real possibility that a newline character in the middle of the sentence in the file will prevent you from finding that sentence directly. Even specifying the newline character in the search pattern will not alleviate this problem. Some text editors and productivity applications simply wrap words on lines without placing a newline character, so searching is not pointless in these cases, but it is an important limitation to keep in mind.

To get details about the regular expression implementation on your specific machine, check the regex and re_format manpages. It is important to note, however, that not all the functions and abilities of regular expressions are built-in to grep. For instance, search and replace is not available. More importantly, there are some useful escape characters that seem to be missing by default.

For instance, \d is an escape sequence to match any numeric character (0 through 9) in some regular expressions. However, this does not seem to be available with grep under standard distribution and compile options (with the exception of Perl-style grep, to be covered later). This guide attempts to cover what is available by default in a standard installation and attempts to be the authoritative resource on the abilities and limits of grep.

The grep program is actually a package of four different pattern-matching programs that use different regular-expression models. Each pattern-matching system has its strengths and weaknesses, and each will be discussed in detail in the following sections. We’ll start with the original model, which we’ll call basic grep.

Basic Regular Expressions (grep or grep -G)

This section focuses on basic grep. Most of the flags for basic grep apply equally to the other versions, which we’ll discuss later.

Basic grep, or grep -G, is the default pattern matching type that is used when calling grep. grep interprets the given set of patterns as a basic regular expression when it executes the command. This is the default grep program that is called, so the -G option is almost always redundant.

Like any command, grep comes with a handful of options that control both the matches found and the way grep displays the results. The GNU version of grep offers most of the options listed in the following subsections.

Match Control

-e pattern, --regexp=pattern

grep -e -style doc.txt

Ensures that grep recognizes the pattern as the regular expression argument. Useful if the regular expression begins with a hyphen, which makes it look like an option. In this case, grep will look for lines that match “-style”.

-f file, --file=file

grep -f pattern.txt searchhere.txt

Takes patterns from file. This option allows you to input all the patterns you want to match into a file, called pattern.txt here. Then, grep searches for all the patterns from pattern.txt in the designated file searchhere.txt. The patterns are additive; that is, grep returns every line that matches any pattern. The pattern file must list one pattern per line. If pattern.txt is empty, nothing will match.

-i, --ignore-case

grep -i 'help' me.txt

Ignores capitalization in the given regular expressions, either via the command line or in a file of regular expressions specified by the -f option. The example here would search the file me.txt for a string “help” with any iteration of lower- and uppercase letters in the word (“HELP”, “HelP”, etc.). A similar but obsolete synonym to this option is -y.

-v, --invert-match

grep -v oranges filename

Returns lines that do not match, instead of lines that do. In this case, the output would be every line in filename that does not contain the pattern “oranges”.

-w, --word-regexp

grep -w 'xyz' filename

Matches only when the input text consists of full words. In this example, it is not enough for a line to contain the three letters “xyz” in a row; there must actually be spaces or punctuation around them. Letters, digits, and the underscore character are all considered part of a word; any other character is considered a word boundary, as are the start and end of the line. This is the equivalent of putting \b at the beginning and end of the regular expression.

-x, --line-regexp

grep -x 'Hello, world!' filename

Like -w, but must match an entire line. This example matches only lines that consist entirely of “Hello, world!”. Lines that have additional content will not be matched. This can be useful for parsing logfiles for specific content that might include cases you are not interested in seeing.

General Output Control

-c, --count

grep -c contact.html access.log

Instead of the normal output, you receive just a count of how many lines matched in each input file. In the example here, grep will simply return the number of times the contact.html file was accessed through a web server’s access log.

grep -c -v contact.html access.log

This example returns a count of all the lines that do not match the given string. In this case, it would be every time someone accessed a file that wasn’t contact.html on the web server.

--color[=WHEN], --colour[=WHEN]

grep -color[=auto] regexp filename

Assuming the terminal can support color, grep will colorize the pattern in the output. This is done by surrounding the matched (nonempty) string, matching lines, context lines, filenames, line numbers, byte offsets, and separators with escape sequences that the terminal recognizes as color markers. Color is defined by the environment variable GREP_COLORS (discussed later). WHEN has three options: never, always, and auto.

-l, --files-with-matches

grep -l "ERROR:" *.log

Instead of normal output, prints just the names of input files containing the pattern. As with -L, the search stops on the first match. If an administrator is simply interested in the filenames that contain a pattern without seeing all the matching lines, this option performs that function. This can make grep more efficient by stopping the search as soon as it finds a matching pattern instead of continuing to search an entire file. This is often referred to as “lazy matching.”

-L, --files-without-match

grep -L 'ERROR:' *.log

Instead of normal output, prints just the names of input files that contain no matches. For instance, the example prints all the logfiles that contain no reports of errors. This is an efficient use of grep because it stops searching each file once it finds any match, instead of continuing to search the entire file for multiple matches.

-m NUM, --max-count=NUM

grep -m 10 'ERROR:' *.log

This option tells grep to stop reading a file after NUM lines are matched (in this example, only 10 lines that contain “ERROR:”). This is useful for reading large files where repetition is likely, such as logfiles. If you simply want to see whether strings are present without flooding the terminal, use this option. This helps to distinguish between pervasive and intermittent errors, as in the example here.

-o, --only-matching

grep -o pattern filename

Prints only the text that matches, instead of the whole line of input. This is particularly useful when implementing grep to examine a disk partition or a binary file for the presence of multiple patterns. This would output the pattern that was matched without the content that would cause problems for the terminal.

-q, --quiet, --silent

grep -q pattern filename

Suppresses output. The command still conveys useful information because the grep command’s exit status (0 for success if a match is found, 1 for no match found, 2 if the program cannot run because of an error) can be checked. The option is used in scripts to determine the presence of a pattern in a file without displaying unnecessary output.

-s, --no-messages

grep -s pattern filename

Silently discards any error messages resulting from nonexistent files or permission errors. This is helpful for scripts that search an entire filesystem without root permissions, and thus will likely encounter permissions errors that may be undesirable. On the other side, it also will suppress useful diagnostic information, which could mean that problems may not be discovered.

Output Line Prefix Control

-b, --byte-offset

grep -b pattern filename

Displays the byte offset of each matching text instead of the line number. The first byte in the file is byte 0, and invisible line-terminating characters (the newline in Unix) are counted. Because entire lines are printed by default, the number displayed is the byte offset of the start of the line. This is particularly useful for binary file analysis, constructing (or reverse-engineering) patches, or other tasks where line numbers are meaningless.

grep -b -o pattern filename

A -o option prints the offset along with the matched pattern itself and not the whole matched line containing the pattern. This causes grep to print the byte offset of the start of the matched string instead of the matched line.

-H, --with-filename

grep -H pattern filename

Includes the name of the file before each line printed, and is the default when more than one file is input to the search. This is useful when searching only one file and you want the filename to be contained in the output. Note that this uses the relative (not absolute) paths and filenames.

-h, --no-filename

grep -h pattern *

The opposite of -H. When more than one file is involved, it suppresses printing the filename before each output. It is the default when only one file or standard input is involved. This is useful for suppressing filenames when searching entire directories.

--label=LABEL

gzip -cd file.gz | grep --label=LABEL pattern

When the input is taken from standard input (for instance, when the output of another file is redirected into grep), the --label option will prefix the line with LABEL. In this example, the gzip command displays the contents of the uncompressed file inside file.gz and then passes that to grep.

-n, --line-number

grep -n pattern filename

Includes the line number of each line displayed, where the first line of the file is 1. This can be useful in code debugging, allowing you to go into the file and specify a particular line number to start editing.

-T, --initial-tab

grep -T pattern filename

Inserts a tab before each matching line, putting the tab between the information generated by grep and the matching lines. This option is useful for clarifying the layout. For instance, it can separate line numbers, byte offsets, labels, etc., from the matching text.

-u, --unix-byte-offsets

grep -u -b pattern filename

This option only works under the MS-DOS and Microsoft Windows platforms and needs to be invoked with -b. This option will compute the byte-offset as if it were running under a Unix system and strip out carriage return characters.

-Z, --null

grep -Z pattern filename

Prints an ASCII NUL (a zero byte) after each filename. This is useful when processing filenames that may contain special characters (such as carriage returns).

Context Line Control

-A NUM, --after-context=NUM

grep -A 3 Copyright filename

Offers a context for matching lines by printing the NUM lines that follow each match. A group separator (--) is placed between each set of matches. In this case, it will print the next three lines after the matching line. This is useful when searching through source code, for instance. The example here will print three lines after any line that contains “Copyright”, which is typically at the top of source code files.

-B NUM, --before-context=NUM

grep -B 3 Copyright filename

Same concept as the -A NUM option, except that it prints the lines before the match instead of after it. In this case, it will print the three lines before the matching line. This is useful when searching through source code, for instance. The example here will print three lines before any line that contains “Copyright”, which is typically at the top of source code files.

-C NUM, -NUM, --context=NUM

grep -C 3 Copyright filename

The -C NUM option operates as if the user entered both the -A NUM and -B NUM options. It will display NUM lines before and after the match. A group separator (--) is placed between each set of matches. In this case, three lines above and below the matching line will be printed. Again, this is useful when searching through source code, for instance. The example here will print three lines before and after any line that contains “Copyright”, which is typically at the top of source code files.

File and Directory Selection

-a, --text

grep -a pattern filename

Equivalent to the --binary-files=text option, allowing a binary file to be processed as if it were a text file.

--binary-files=TYPE

grep --binary-files=TYPE pattern filename

TYPE can be either binary, without-match, or text. When grep first examines a file, it determines whether the file is a “binary” file (a file primarily composed of non-human-readable text) and changes its output accordingly. By default, a match in a binary file causes grep to display simply the message “Binary file somefile.bin matches.” The default behavior can also be specified with the --binary-files=binary option.

When TYPE is without-match, grep does not search the binary file and proceeds as if it had no matches (equivalent to the -l option). When TYPE is text, the binary file is processed like text (equivalent to the -a option). When TYPE is without-match, grep will simply skip those files and not search through them. Sometimes --binary-files=text outputs binary garbage and the terminal may interpret some of that garbage as commands, which in turn can render the terminal unreadable until reset. To recover from this, use the commands tput init and tput reset.

-D ACTION, --devices=ACTION

grep -D read 123-45-6789 /dev/hda1

If the input file is a special file, such as a FIFO or a socket, this flag tells grep how to proceed. By default, grep will process these files as if they were normal files on a system. If ACTION is set to skip, grep will silently ignore them. The example will search an entire disk partition for the fake Social Security number shown. When ACTION is set to read, grep will read through the device as if it were a normal file.

-d ACTION, --directories=ACTION

grep -d ACTION pattern path

This flag tells grep how to process directories submitted as input files. When ACTION is read, this reads the directory as if it were a file. recurse searches the files within that directory (same as the -R option), and skip skips the directory without searching it.

--exclude=GLOB

grep --exclude=PATTERN path

Refines the list of input files by telling grep to ignore files whose names match the specified pattern. PATTERN can be an entire filename or can contain the typical “file-globbing” wildcards the shell uses when matching files (*, ? and []). For instance, --exclude=*.exe will skip all files ending in .exe.

--exclude-from=FILE

grep --exclude-from=FILE path

Similar to the --exclude option, except that it takes a list of patterns from a specified filename, which lists each pattern on a separate line. grep will ignore all files that match any lines in the list of patterns given.

--exclude-dir=DIR

grep --exclude-dir=DIR pattern path

Any directories in the path matching the pattern DIR will be excluded from recursive searches. In this case, the actual directory name (relative name or absolute path name) has to be included to be ignored. This option also must be used with the -r option or the -d recurse option in order to be relevant.

-l

grep -l pattern filename

Same as the --binary-files=without-match option. When grep finds a binary file, it will assume there is no match in the file.

--include=GLOB

grep --include=*.log pattern filename

Limits searches to input files whose names match the given pattern (in this case, files ending in .log). This option is particularly useful when searching directories using the -R option. Files not matching the given pattern will be ignored. An entire filename can be specified, or can contain the typical “file-globbing” wildcards the shell uses when matching files (*, ? and []).

-R, -r, --recursive

grep -R pattern path
grep -r pattern path

Searches all files underneath each directory submitted as an input file to grep.

Other Options

--line-buffered

grep --line-buffered pattern filename

Uses line buffering for the output. Line buffering output usually leads to a decrease in performance. The default behavior of grep is to use unbuffered output. This is generally a matter of preference.

--mmap

grep --mmap pattern filename

Uses the mmap() function instead of the read() function to process data. This can lead to a performance improvement but may cause errors if there is an I/O problem or the file shrinks while being searched.

-U, --binary

grep -U pattern filename

An MS-DOS/Windows-specific option that causes grep to treat all files as binary. Normally, grep would strip out carriage returns before doing pattern matching; this option overrides that behavior. This does, however, require you to be more thoughtful when writing patterns. For instance, if content in a file contains the pattern but has a newline character in the middle, a search for that pattern will not find the content.

-V, --version

Simply outputs the version information about grep and then exits.

-z, --null-data

grep -z pattern

Input lines are treated as though each one ends with a zero byte, or the ASCII NUL character, instead of a newline. Similar to the -Z or --null options, except this option works with input, not output.

One final limitation of basic grep: the “extended” regular expressions metacharacters—?, +, {, }, |, (, and )—do not work with basic grep. The functions provided by those characters exist if you preface them with an escape. More on that in the next section.

Extended Regular Expressions (egrep or grep -E)

grep -E and egrep are the same exact command. The commands search files for patterns that have been interpreted as extended regular expressions. An extended regular expression goes beyond just using the previously mentioned options; it uses additional metacharacters to create more complex and powerful search strings. As far as command-line options, grep -E and grep take the same ones—the only differences are in how they process the search pattern:

?

? in an expression carries the meaning of optional. Any character preceding the question mark may or may not appear in the target string. For example, say you are looking for the word “behavior”, which can also be written as “behaviour”. Instead of using the or (|) option, you can use the command:

egrep 'behaviou?r' filename

As a result, the search is successful for both “behavior” and “behaviour” because it will treat the presence or absence of the letter “u” the same way.

+

The plus sign will look at the previous character and allow an unlimited amount of repetitions when it looks for matching strings. For instance, the following command would match both “pattern1” and “pattern11111”, but would not match “pattern”:

egrep 'pattern1+' filename

{n,m}

The braces are used to determine how many times a pattern needs to be repeated before a match occurs. For instance, instead of searching for “patternnnn”, you could enter the following command:

egrep 'pattern{4}' filename

This will match any string that contains “patternnnn” without going through the trouble of typing out repeated strings. In order to match at least four repetitions, you would use the following command:

egrep 'pattern{4,}' filename

On the other hand, look at the following example:

egrep 'pattern{,4}' filename

Despite the fact that it would fit in with the conventions already used, this is not valid. The command just shown would result in no matches because the ability to have “no more than X” matches is not available.

To match between four and six repetitions, use the following:

egrep 'pattern{4,6}' filename

|

Used in a regular expression, this character signifies “or.” As a result, pipe (|) allows you to combine several patterns into one expression. For example, suppose you need to find either of two names in file. You could issue the following command:

egrep 'name1|name2' filename

It would match on lines containing either “name1” or “name2”.

( )

Parentheses can be used to “group” particular strings of text for the purposes of backreferences, alternation, or simply readability. Additionally, the use of parentheses can help resolve any ambiguity in precisely what the user wants the search pattern to do. Patterns placed inside parentheses are often called subpatterns.

Also parentheses put limits on pipe (|). This allows the user to more tightly define which strings are part of or in scope of the “or” operation. For instance, to search for lines that contain either “pattern” or “pattarn”, you would use the following command:

egrep 'patt(a|e)rn' filename

Without the parentheses, the search pattern would be patta|ern, which would match if the string “patta” or “ern” is found, a very different outcome than the intention.

In basic regular expressions, the backslash (\) negates the metacharacter’s behavior and forces the search to match the character in a literal sense. The same happens in egrep, but there is an exception. The metacharacter { is not supported by the traditional egrep. Although some versions interpret \{ literally, it should be avoided in egrep patterns. Instead, [{] should be used to match the character without invoking the special meaning.

It is not precisely true that basic grep does not have these metacharacters as well. It does, but they cannot be used directly. Each of the special metacharacters in extended regular expressions needs to be prefaced by an escape to draw out its special meaning. Note that this is the reverse of normal escaping behavior, which usually strips special meaning.

Table 1-3 illustrates how to use the extended regular expressions metacharacters with basic grep.

Table 1-3. Basic versus extended regular expressions comparison

Basic regular expressions

Extended regular expressions

'\(red\)'

'(red)'

'a\{1,3\}'

'a{1,3}'

'behaviou\?r'

'behaviou?r'

'pattern\+'

'pattern+'

From Table 1-3, you get the idea why people would prefer to just use extended grep when they want to use extended regular expressions. Convenience aside, it is also easy to forget to place a necessary escape in basic regular expressions, which would cause the pattern to silently not return any matches. An ideal regular expression should be clear and use as few characters as possible.

Fixed Strings (fgrep or grep -F)

In the following section, we discuss grep -F, or fgrep. fgrep is known as fixed string or fast grep. It is known as “fast grep” because of the great performance it has compared to grep and egrep. It accomplishes this by dropping regular expressions altogether and looking for a defined string pattern. It is useful for searching for specific static content in a precise manner, similar to the way Google operates.

The command to evoke fgrep is:

fgrep string_pattern filename

By design, fgrep was intended to operate fast and free of intensive functions; as a result, it can take a more limited set of command-line options. The most common ones are:

-b

fgrep -b string_pattern filename

Shows the block number where the string_pattern was found. Because entire lines are printed by default, the byte number displayed is the byte offset of the start of the line.

-c

fgrep -c string_pattern filename

This counts the number of lines that contain one or more instances of the string_pattern.

-e, -string

fgrep -e string_pattern filename

Used for the search of more than one pattern or when the string_pattern begins with hyphen. Though you can use a newline character to specify more than one string, instead you could use multiple -e options, which is useful in scripting:

fgrep -e string_pattern1
-e string_pattern2 filename

-f file

fgrep -f newfile string_pattern filename

Outputs the results of the search into a new file instead of printing directly to the terminal. This is unlike the behavior of the -f option in grep; there it specifies a search pattern input file.

-h

fgrep -h string_pattern filename

When the search is done in more than one file, using -h stops fgrep from displaying filenames before the matched output.

-i

fgrep -i string_pattern filename

The -i option tells fgrep to ignore capitalization contained in the string_pattern when matching the pattern.

-l

fgrep -l string_pattern filename

Displays the files containing the string_pattern but not the matching lines themselves.

-n

fgrep -n string_pattern filename

Prints out the line number before the line that matches the given string_pattern.

-v

fgrep -v string_pattern filename

Matches any lines that do not contain the given string_pattern.

-x

fgrep -x string_pattern filename

Prints out the lines that match the string_pattern in their entirety. This is the default behavior of fgrep, so usually it does not need to be specified.

Perl-Style Regular Expressions (grep -P)

Perl-style regular expressions use the Perl-Compatible Regular Expressions (PCRE) library to interpret the pattern and perform searches. As the name implies, this style uses Perl’s implementation of regular expressions. Perl has an advantage because the language was optimized for text searching and manipulation. As a result, PCRE can be more efficient and far more function-rich for finding content. The consequence is that it can be horribly messy and complex. To put it another way, using PCRE to find information is like using a weed whacker on yourself to do brain surgery: it gets the job done with minimum of effort, but it is an awful mess.

The specific search features and options with PCRE are not dependent upon grep itself, but use the libpcre library and the underlying version of Perl. This means that it can be highly variable between machines and operating systems. Usually the pcrepattern or pcre manpages will provide machine-specific information on the options that are available on your machine. What follows is a general set of PCRE search functions that should be available on most machines.

Also note that Perl-style regular expressions may or may not be present by default on your operating system. Fedora and Red Hat–based systems tend to include them (assuming you install the PCRE library), but Debian, for instance, does not enable Perl-style regular expressions by default in their grep package. Instead, they ship a pcregrep program, which provides very similar functionality to grep -P. Individuals can, of course, compile their own grep binary that does include PCRE support should they be so inclined.

To test whether Perl-style regular expression support is built-in to your version of grep, run the following command (or something like it):

$ grep -P test /bin/ls
grep: The -P option is not supported

This usually means that when grep was built it could not find the libpcre library or that it was intentionally disabled with the --disable-perl-regexp configuration option when it was compiled. The solution is to either install libpcre and recompile grep or find an applicable package for your operating system.

The general form of using Perl-style grep is:

grep -P options pattern file

It is important to note that, unlike grep -F and grep -E, there is no “pgrep” command. The pgrep command is used to search for running processes on a machine. All the same command-line options that are present for grep will work with grep -P; the only difference is how the pattern is processed. PCRE provides additional metacharacters and character classes that can be used enhance search functionality. Other than the additional metacharacters and classes, the pattern is constructed in the same way as a typical regular expression.

This section covers only four aspects of PCRE options: character types, octal searching, character properties, and PCRE options.

Character Types

Although there is some overlap here with standard grep, PCRE comes with its own set of escapes that provide a more robust set of matching. Table 1-4 contains the list of escapes available under PCRE.

Table 1-4. PCRE-specific escapes

\a

Matches the “alarm” character (HEX 07)

\cX

Matches Ctrl-X, where X is any letter

\e

Matches escape character (HEX 1B)

\f

Matches form feed character (HEX 0C)

\n

Matches newline character (HEX 0A)

\r

Matches carriage return (HEX 0D)

\t

Matches tab character (HEX 09)

\d

Any decimal digit

\D

Any non-decimal character

\s

Any whitespace character

\S

Any non-whitespace character

\w

Any “word” character

\W

Any “non-word” character

\b

Matches when at word boundary

\B

Matches when not at word boundary

\A

Matches when at start of subject

\Z

Matches when at end of subject or before newline

\z

Matches when at end of subject

\G

Matches at first matching position

Octal Searching

To search for octal charters, use the / metacharacter followed by the octal number of the metacharacter. For instance, to search for “space”, use /40 or /040. However, this is one of the areas where PCRE can be ambiguous if you aren’t careful. The / metacharacter can also be used for backreference (a reference to a previous pattern given to PCRE).

For instance, /1 is a backreference to the first pattern in a list, not octal character 1. To be free of ambiguity, the easiest way is to specify the octal character as a three-digit number. Up to 777 is permitted in UTF-8 mode. All single-digit numbers given after the slash are interpreted as a backreference, and if there have been more than XX patterns, then \XX is interpreted as a backreference as well.

Additionally, PCRE can search from a character in hex format or a string of characters represented in hex format. \x0b will search for the hex character 0b, for example. To search for a hex string, simply use \x{0b0b....}, where the string is contained within the braces.

Character Properties

Additionally, PCRE comes with a set of functions that will search for characters based on their property. This comes in two particular flavors, language and character type. To use this, the \p or \P sequence is used. \p searches if a given property is present, whereas \P matches any character where it is not present.

To search for the presence (or absence) of characters that belong to a certain language, for example, you would use \p{Greek} to find Greek characters. \P{Greek}, on the other hand, would match any character that is not part of the Greek character set. For a complete list of languages available, consult the manpage for the particular pcrepattern implementation on your system.

The other set of properties refers to the attributes of a given character (uppercase, punctuation, etc.). The capital letter represents the major grouping of characters, and the small letter refers to the subgroup. If only the capital letter is specified (e.g., /p{L}), all subgroups are matched. Table 1-5 shows the complete list of property codes.

Table 1-5. PCRE character properties

C | Other | No | Other number
Cc | Control | P | Punctuation
Cf | Format | Pc | Connector punctuation
Cn | Unassigned | Pd | Dash punctuation
Co | Private use | Pe | Close punctuation
Cs | Surrogate | Pf | Final punctuation
L | Letter | Pi | Initial punctuation
Ll | Lowercase | Po | Other punctuation
Lm | Modifier | Ps | Open punctuation
Lo | Other letter | S | Symbol
Lt | Title case | Sc | Currency symbol
Lu | Uppercase | Sk | Modifier symbol
M | Mark | Sm | Mathematical symbol
Mc | Spacing mark | So | Other symbol
Me | Enclosing mark | Z | Separator
Mn | Non-spacing mark | Zl | Line separator
N | Number | Zp | Paragraph separator
Nd | Decimal | Zs | Space separator
Nl | Letter number |  |

These properties allows for creating more robust patterns with fewer characters based on a large number of properties. One important note, however: if pcre is compiled by hand, the --enable-unicode-properties configuration option must be used to compile in support for these options. Some libpcre packages (i.e., Fedora or Debian packages) have this built-in (especially internationally minded ones), but others do not. To check whether support is built-in to pcre, run the following (or something like it):

$ grep -P '\p{Cc}' /bin/ls
grep: support for \P, \p, and \X has not been compiled

That error message about support being compiled in has to do with pcre and not grep, which is not exactly intuitive. The solution is to either find a better package or compile your own with the correct options.

PCRE Options

Finally, there are four different options that can alter the way PCRE looks for text: PCRE_CASELESS (i), PCRE_MULTILINE (m), PCRE_DOTALL (s), and PCRE_EXTENDED (x). PCRE_CASELESS will match patterns regardless of differences in capitalization. By default, PCRE treats a line of text as one line, even if several \n characters are present. PCRE_MULTILINE will allow for treating those \n characters as lines, so if $ or ^ is used, it will search lines based on the presence of \n and actual hard lines in the search string.

PCRE_DOTALL causes PCRE to interpret the . (dot) metacharacter to include newlines when it does “wildcard” matching. PCRE_EXTENDED is useful for including comments (placed within unescaped # characters) in complicated search strings.

To enable these options, place the given option letter inside parentheses with a beginning question mark. For instance, to craft a pattern that will search for the word “copyright” in a caseless format, you would use the following pattern:

'(?i)copyright'

Any combination of letters can be used inside the parentheses. These options can be placed so that they operate on only part of the search string: simply place them at the beginning of the part of the string where the option should take effect. To negate an option, preface the letter with a - (hyphen). For example:

'Copy(?i)righ(?-i)t'

This would match “CopyRIGHt”, “CopyrIgHt”, and “Copyright”, but it would not match “COPYright” or “CopyrighT”.

'(?imsx)copy(?-sx)right'

This would set all the PCRE options we’ve discussed, but once it reaches the “r” character, PCRE_DOTALL and PCRE_EXTENDED would be turned off.

If using Perl-based regular expressions seems complicated, that’s because it is. There is far more to it than can be discussed here, but the chief advantage with PCRE is its flexibility and power, which goes well beyond what regular expressions can do. The downside is the great deal of complexity and ambiguity that can be involved.

Introduction to grep-Relevant Environment Variables

In previous examples, we came across the concept of environment variables and their effect on grep. Environment variables allow you to customize the default options and behavior of grep by defining the environment settings of the shell, thereby making your life easier. Issue an env command in a terminal to output all the current parameters. The following is an example of what you might see:

$ env
USER=user
LOGNAME=user
HOME=/home/user
PATH=/usr/local/sbin:/usr/local/bin:/usr
/sbin:/usr/bin:/sbin:/bin:/usr/X11R6/bin:.
SHELL=/usr/local/bin/tcsh
OSTYPE=linux
LS_COLORS=no=0:fi=0:di=36:ln=33:ex=32
:bd=0:cd=0:pi=0:so=0:do=0:or=31
VISUAL=vi
EDITOR=vi
MANPATH=/usr/local/man:/usr/man:/usr
/share/man:/usr/X11R6/man
...

By manipulating the .profile file in your home directory, you can make permanent changes to the variables. For example, using the output just shown, suppose you decide to change your EDITOR from vi to vim. In .profile, type:

setenv EDITOR vim

After writing out the changes, this permanently ensures vim will be the default editor for each session that uses this .profile. The previous examples use some of the built-in variables, but if you are code-savvy, there is no limit (save for your imagination) on the variables you create and set.

To reiterate, grep is a powerful search tool because of the many options available to the user. Variables are no different. There are several specific options, which we describe in detail later. However, it should be noted that grep falls back onto C locale when the variables LC_foo, LC_ALL, or LANG are not set, when the local catalog is not installed, or when the national language support (NLS) is not complied.

To start off, “locale” is the convention used for communicating in a particular language. For example, when you set the variable LANG or language to English, you are using the conventions tied in with the English language for interacting with the system. When the computer starts up, it defaults to the conventions set up in the kernel, but these settings can be changed.

LC_ALL is not actually a variable, but rather a macro that allows you to “set locale” for all purposes. Although LC_foo is a locale-specific setting for a variety of character sets, foo can be replaced by ALL, COLLATE, CTYPE, MONETARY, or TIME, to name a few. These are then set to create the overall language conventions for the environment, but it becomes possible to use one language’s conventions for money and another for time conventions.

How is this related to grep? For instance, many of the POSIX character classes depend on which specific locale is being used. PCRE also borrows heavily from locale settings, especially for the character classes it uses. Because grep is designed for searching for text in text files, the way language is processed on a machine matters, and that is determined by the locale.

For most users, leaving the locale settings as the default is fine. Users who wish to search in other languages or want to work in a different language than the system environment might want to change these.

Now that we have familiarized ourselves with the concept of the locale, the environment variables specific to grep are:

GREP_OPTIONS

This variable overrides the “compiled” default options for grep. This is as if they were placed in the command line as options themselves. For example, suppose you want to create a variable for the option --binary-files and set it to text. Therefore, --binary-files automatically implies --binary-files=text, without the need to write it out. However, --binary-files can be overridden by specific and different variables (--binary-files=without-match, for example).

This option is especially useful for scripting, where a set of “default options” can be specified once in the environment variables and it never has to be referenced again. There is one gotcha, though: any options set with GREP_OPTIONS will be interpreted as though they were put on the command line. This means that command-line options don’t override the environment variable, and if they conflict, grep will produce an error. For instance:

$ export GREP_OPTIONS=-E
$ grep -G '(red)' test
grep: conflicting matchers specified

Some care and consideration needs to be taken when putting options into this environment variable, and it is almost always best to set only those options that are of a general nature (for instance, how to handle binary files or devices, whether to use color, etc.).

GREP_COLORS (or GREP_COLOR for older versions)

This variable specifies the color to be used for highlighting the matching pattern. This is invoked with the --color[=WHEN] option, where WHEN is never, auto, or always. The setting should be a two-digit number from the list in Table 1-6 that corresponds to the specific color.

Table 1-6. List of color options

Color

Color code

Black

0;30

Dark gray

1;30

Blue

0;34

Light blue

1;34

Green

0;32

Light green

1;32

Cyan

0;36

Light cyan

1;36

Red

0;31

Light red

1;31

Purple

0;35

Light purple

1;35

Brown

0;33

Yellow

1;33

Light gray

0;37

White

1;37

The colors need to be specified in a particular syntax because the highlighting covers additional fields as well, not just matching words. The default setting is as follows:

GREP_COLORS='ms=01;31:mc=01;31:sl=:
cx=:fn=35:ln=32:bn=32:se=36'

If the desired color starts with 0 (as with 0;30, which is black), the 0 can be discarded to shorten the setting. Where settings are left blank, the default terminal color normal text is used. ms stands for matching string (i.e., the pattern you enter), mc is matching context (i.e., lines shown with the -C option), sl is for the color of selected lines, cx is the color for selected context, fn is the color for the filename (when shown), ln is the color for line numbers (when shown), bn is for byte numbers (when shown), and se is for separator color.

LC_ALL, LC_COLLATE, LANG

These variables have to be specified in that order, but ultimately they determine the collating or an arrangement in the proper sequence of the expressed ranges. For instance, this could be the sequence of letters for alphabetizing.

LC_ALL, LC_CTYPE, LANG

These variables determine LC_CTYPE, or the type of characters to be used by grep. For example, which characters are whitespace, which will be a form feed, and so on.

LC_ALL, LC_MESSAGES, LANG

These variables determine the MESSAGES locale and which language grep will use for the messages that it outputs. This is a prime example of where grep falls back on the C locale’s default, which is American English.

POSIXLY_CORRECT

When set, grep follows the POSIX.2 requirements that state any options following filenames are treated as filenames themselves. Otherwise, those options are treated as if they are moved before filenames and treated as options. For instance, grep -E string filename -C 3 would interpret “-C 3” as filenames instead of as an option. Additionally, POSIX.2 states that any unrecognized options be labeled as “illegal”—by default, under GNU grep, these are treated as “invalid.”

Choosing Between grep Types and Performance Considerations

Now that we have gone over all four grep programs, the question is how should you determine which to employ for a given task. For most routine uses, people tend to use the standard grep command (grep -G) because performance isn’t an issue when searching small files and when complex search patterns aren’t necessary. Generally, the basic grep is the default choice for most people, and so the question becomes when it makes sense to use something else.

When to Use grep -E

Although almost everything can be done in grep -G that can be done in grep -E, the latter has the advantage of accomplishing the task in fewer characters, without the counterintuitive escaping discussed earlier. All of the extra functionality in extended regular expressions has to do with quantifiers or subpatterns. Additionally, if any significant use of backreferences is needed, extended regular expressions are ideal.

When to Use grep -F

There is one prerequisite to using grep -F, and if a user cannot meet that requirement, grep -F is simply not an option. Namely, any search pattern for grep -F cannot contain any metacharacters, escapes, wildcards, or alternations. Its performance is faster, but at the expense of functionality.

That said, grep -F is extremely useful for quickly searching large amounts of data for tightly defined strings, making it the ideal tool to search through immense logfiles quickly. In fact, it is fairly easy to develop a robust “log watching” script with grep -F and a good text file listing of important words or phrases that should be pulled out of logfiles for analysis.

Another good use for grep -F is searching through mail logs and mail folders to ensure delivery of emails to users, especially on systems with many mail accounts. This is made possible by assigning every email message a unique Message ID. For instance:

grep -FHr MESSAGE-ID /var/mail

This command will search for the fixed string MESSAGE-ID for all files inside /var/mail (and recurse any subdirectories), and then display the match and also the filename. This is a quick, down-and-dirty way to see which users have a particular message sitting in their mailbox. The real bonus is that this information can be verified without ever having to look inside a user’s mailbox and deal with the privacy issues of reading other people’s mail. In reality, you may wish to search mailbox directories and spam folders, which typically aren’t stored under /var/mail, but you get the point of how this works.

When to Use grep -P

Perl-style regular expressions are hands-down the most powerful of all the styles presented in this book. They are also the most complicated, prone to user-error, and potentially capable of bogging down a system’s performance if not done correctly. However, it is clearly the superior style out of all the regular expression formats used in this book.

For this reason, many applications prefer to use PCRE instead of GNU regular expressions. For instance, the popular intrusion detection system snort uses PCRE to match bad packets on the wire. The patterns are written intelligently so that there can be very little packet loss, even though a single machine can search all the packets going through a fully loaded 100 MB or GB interface. As has been said before, writing a regular expression well tends to be more important than the particular regular expression format you use.

Some people simply prefer to use grep -P as their default (for instance, by specifying -P inside their GREP_OPTIONS environment variable). If searching is going to be done in an “international” way, the PCRE language character classes make this far easier. PCRE comes with a many more character classes for finely tuning a regular expression, beyond what is possible with the POSIX definitions, for instance. Most importantly, the ability to use the various PCRE options (e.g., PCRE_MULTILINE) allows searching in more powerful ways than GNU regular expressions.

For simple to moderately complex regular expressions, grep -E suffices. However, there are limitations, and those may push a user toward PCRE. It is a trade-off between complexity and functionality. PCRE also helps users craft regular expressions that can be almost immediately transferred directly into Perl scripts (or transferred from Perl scripts) without having to go through a great deal of translation.

Performance Implications

For most routine uses, grep performance is not an issue. Even megabyte-long files can be searched quickly using any of the specific grep programs without any noticeable performance difference. Obviously, the larger the file, the longer the search takes. For searching through gigabytes or terabytes of data when performance is a consideration, grep -F is likely the desired solution, but only if it is possible to craft the search pattern without using any metacharacters, alternations, or backreferences. This is not always the case.

The more “choices” given to grep, the longer a particular search takes. For instance, using many alternations causes grep to search lines multiple times instead of just once. This may be necessary for a given search pattern, but occasionally alternation can be rewritten as a character class. For instance:

grep -E '(0|2|4|6|8)' filename
grep -E '[02468]' filename

Comparing the two examples, the second one performs better because no alternation is used and so lines do not have to be searched multiple times. Avoid alternation when other alternatives exist that accomplish the same thing.

By far, the biggest cause of performance slowdowns when using grep is the use of backreferences. The time it takes grep to run a command increases almost exponentially with the use of additional backreferences. Backreferences can, in effect, become handy aliases to previous subpatterns; however, performance will suffer. Backreferences should not be used if performance is a concern and when subpatterns are not using alternation for this reason. See the next section for a more detailed discussion of backreferences.

In closing, among grep -G, grep -E, and grep -P, there is not much of a performance impact; it depends mostly on how the regular expression itself is constructed. That said, grep -P provides the most opportunities for slower performance but also the most flexibility to match a wide variety of content.

Advanced Tips and Tricks with grep

As mentioned earlier, grep can be used in very powerful ways to search for content in files or across a filesystem. It is possible to use previous matches to search later strings (called backreferences). There are also a variety of tricks to search nonpublic personal information and even find binary strings in binary files. The following sections discuss some advanced tips and tricks.

Backreferences

The grep program has the ability to match based on multiple previous conditions. For instance, if you want to find all lines that repeatedly use a particular set of words, a single grep pattern will not work; however, it is possible to do this with the use of backreferences.

Suppose you wish to find any line that has multiple instances of the words “red”, “blue”, or “green”. Imagine the following text file:

The red dog fetches the green ball.
The green dog fetches the blue ball.
The blue dog fetches the blue ball.

Only the third line repeats the use of the same color. A regular expression pattern of ''(red|green|blue)*(red|green|blue)'' would return all three lines. To overcome this problem, you could use backreferences:

grep -E '(red|green|blue).*\1' filename

This command matches only the third line, as intended. For extended regular expressions, only a single digit can be used to specify a backreference (i.e., you can only refer back to the ninth backreference). Using Perl-style regular expressions, theoretically you can have many more (at least two digits).

This could be used to validate XML syntax (i.e., the “opening” and “closing” tags are the same), HTML syntax (match all lines with the various opening and closing “heading” tags, such as <h1>, <h2>, etc.), or even to analyze writing for pointless repetition of buzzwords.

It is important to note that backreferences require the use of parentheses to determine reference numbers. grep will read the search pattern from left to right, and starting with the first parenthetical subpattern it finds, it will start numbering from 1.

Typically, backreferences are used when a subpattern contains alternation, as in the previous example. It is not required, however, for a subpattern to actually contain alternation. For instance, assuming there is a large subpattern that you wish to refer back to later in the regular expression, you could use a backreference as an artificial “alias” for that subpattern without having to type out the entire pattern multiple times. For instance:

grep -E '(I am the very model of a
modern major general.).*\1' filename

would search for repetitions of the sentence “I am the very model of a modern major general.” separated by any amount of optional content. This certainly reduces the number of keystrokes and makes the regular expression more manageable, but it also causes some performance considerations as discussed previously. The user needs to weigh the benefits of convenience with performance, depending on what she is trying to accomplish.

Binary File Searching

Up to this point, it seems that grep could only be used to search for text strings in text files. This is what it is most used for, but grep can also search for strings in binary files.

It is important to note that “text” files exist on computers mostly for human readability. Computers talk purely in binary and machine code. The entire ASCII character set consists of 255 characters, of which only about 60 are “human-readable.” However, many computer programs contain text strings as well. For instance, “help” screens, filenames, error messages, and expected user input may appear as text inside binary files.

The grep command does not distinguish to any great extent between searching text or binary files. As long as you feed it patterns (even binary patterns), it will happily search any file for the patterns you tell it to search. It does do an initial check to see if a file is binary and alters the way it displays results accordingly (unless you manually specify other behavior):

bash$ grep help /bin/ls
Binary file /bin/ls matches

This command searches for the string “help” in the binary file ls. Instead of showing the line where the text appears, it simply indicates that a match was found. The reason again relates to the fact that computer programs are in binary and therefore not human-readable. There are no “lines” in programs, for instance. Binary files don’t add line breaks because they would alter the code—they are simply a feature to make text files more readable, which is why grep tells you only whether there is a match. To get an idea of the kind of text that is in a binary file, you can use the strings command. For instance, strings /bin/ls would list all the text strings in the ls command.

There is another way to search binary files that is specific for binary data as well. In this case, you need to rely on some tricks, because you cannot type in binary data directly with a normal keyboard. Instead, you need to use a special form of a regular expression to type in the hexadecimal equivalent of the data you want to search. For instance, if you wanted to search a binary file that had a hexadecimal string of ABAA, you would type the following command:

bash$ grep '[\xabaa]' test.hex
Binary file test.hex matches

The general format is to type /x and then the hexadecimal string you wish to match. There is no real limit to the size of the string you can enter. This type of searching could be useful in malware analysis. For instance, the metasploit framework (http://www.metasploit.org) can generate binary payloads to exploit remote machines. This payload could be used to establish a remote shell, add accounts, or accomplish other malicious activity.

Using hexadecimal searching, it would be possible to determine from binary strings which of the metasploit payloads were being used in an actual attack. Additionally, if you could determine a unique hexadecimal string that was used in a virus, you could create a basic virus scanner using grep. In fact, many older virus scanners did more or less this very thing by searching for unique binary strings in files against a list of known bad hexadecimal signatures.

Many buffer overflows or exploit payloads are written in C, and it is typical to write out each hexadecimal digit in C with the \x escape. For instance, take the following buffer overflow exploit payload:

"\xeb\x17\x5e\x89\x76\x08\x31\xc0\x88\x46
\x07\x89\x46\x0c\xb0\x0b\x89\xf3\x8d\x4e
\x08\x31\xd2\xcd\x80\xe8\xe4\xff\xff\xff
\x2f\x62\x69\x6e\x2f\x73\x68\x58";

It may be more advantageous to write the regular expression in the same way instead of typing out the entire hexadecimal string, if for no other reason than to allow for copying and pasting from exploit code. Either way could work—it is your preference. The exploit just shown works against Red Hat 5 and 6 machines (not Enterprise Red Hat), so this particular code is useless, but it is not hard to find exploit code in the wild.

As an interesting aside, this method does not seem to work for searching files that are recognized as text files. For instance, if you tried to search for text in a text file using the hexadecimal equivalent of the ASCII codes, grep would not find the content. Searching for hexadecimal strings works only for files that grep recognizes as “binary.”

Useful Recipes

The following is a quick list of useful grep recipes to find certain classes of content. Because the availability of Perl-based regular expressions varies, the list will use extended grep-style recipes, even though Perl would be quicker in many cases.

Typing these commands in on the command line and returning the default output to the screen may not make much sense if the desire is to search for sensitive information on a partition. It would make more sense to use the -l and -r options to recurse through an entire filesystem and display matching filenames instead of entire lines.

For many recipes, it makes sense to place \b before and after the string. This ensures that the content has some sort of whitespace before and after it, preventing the case in which you are looking for a 9-digit number that appears to be a Social Security number but get false-positive matches on 29-digit numbers.

Finally, these patterns (and potentially others that may make sense) can be put into a file and used as a list of input patterns given to grep, so all of them are searched for at the same time.

IP addresses

$ grep -E '\b[0-9]{1,3}(\.[0-9]{1,3}){3}
\b' patterns

123.24.45.67
312.543.121.1

This pattern will help point out IP addresses in a file. It is important to note that [0–9] could have just as easily been replaced with [:digit:] for better readability. However, most users tend to prefer less keystrokes compared to readability when given the choice. A second note is that this will also find strings that aren’t valid IP addresses, such as the second one listed in the example. Regular expressions work on individual characters, and there is not a good way to tell grep to search for a range of values from 1–255. In this case, there may be false positives. A more complicated formula to ensure that false positives are not registered looks like:

$ grep -E '\b((25[0-5]|2[0-4][0-9]|[01]?
[0-9][0-9]?)\.){3}(25[0-5]|2[0-4][0-9]|
[01]?[0-9][0-9]?)\b' patterns

In this case, it makes sure to find IP addresses with an octet between 0–255 by establishing a combination of patterns that would work. This does guarantee only matching IP addresses, but it is more complicated and has lower performance.

MAC addresses

$ grep -Ei '\b[0-9a-f]{2}
(:[0-9a-f]{2}){5}\b' patterns

ab:14:ed:41:aa:00

In this case, the additional -i option is added so no regard is given to capitalization. As with the IP recipe, [:xdigit:] could be used in place of [0–9a–f] if better readability is desired.

Email addresses

$ grep -Ei '\b[a-z0-9]{1,}@*\.
(com|net|org|uk|mil|gov|edu)\b' patterns

test@some.com
test@some.edu
test@some.co.uk

The list shown here is only a partial subset of top-level domains that are currently approved for use. For instance, one may wish to search for only U.S.-based addresses, so the .uk result may not make much sense. Perhaps identifying obvious spammers in the mail logs is the goal, in which case searching for the .info top-level domain may be advised (we have never met anyone who has gotten legitimate email from that top-level domain). The pattern shown is basically a starting point for customization.

U.S.-based phone numbers

$ grep -E '\b(\(|)[0-9]{3}
(\)|-|\)-|)[0-9]{3}(-|)[0-9]{4}\b'
 patterns

(312)-555-1212
(312) 555-1212
312-555-1212
3125551212

In this case, the pattern is a little more complex because of the wide variety of U.S.-based phone numbers. There may be spaces, dashes, parentheses, or nothing at all. Note the way the parentheses indicate the presence of varying characters, including no character at all.

Social Security numbers

$ grep -E '\b[0-9]{3}( |-|)
[0-9]{2}( |-|)[0-9]{4}\b' patterns

333333333
333 33 3333
333-33-3333

Social Security numbers are the key to an individual’s identity in the United States, so the use of this identifier is becoming increasingly restricted. Many organizations now actively search all files on a system for Social Security numbers using tools such as Spider. This tool is not much more sophisticated than a list of these grep recipes. In this case, however, the pattern is far simpler than the one for phone numbers.

Credit card numbers

For most credit card numbers, this expression works:

$ grep -E '\b[0-9]{4}(( |-|)
[0-9]{4}){3}\b' patterns

1234 5678 9012 3456
1234567890123456
1234-5678-9012-3456

American Express card numbers would be caught by this expression:

$ grep -E '\b[0-9]{4}( |-|)
[0-9]{6}( |-|)[0-9]{5}\b' patterns

1234-567890-12345
123456789012345
1234 567890 12345

There are two versions because American Express uses a different pattern than other credit cards. However, the basic idea remains the same: looking for groupings of numbers that fit the general pattern of a credit card number.

Copyright-protected or confidential material

Finally, many organizations have internal data classifications that make it easy to identify privileged information within an organization. Hopefully, these data classifications come with required text that must be displayed in the document. Those strings can simply be put in as search patterns in grep (or fgrep) to quickly identify where protected information may reside on a disk, especially on those machines where that information does not belong.

Most file formats with text are not true ASCII files, but usually the text content can be located and identified within the file. For instance, you can use grep to search for the presence of certain strings in Word files, even though these files aren’t viewable in a terminal.

For instance, if your corporation uses the tag “ACME Corp.—Proprietary and Confidential,” you could use the following command to locate files that have this content:

fgrep -l 'ACME Corp. -
Proprietary and Confidential' patterns

Searching through large numbers of files

Like many shell commands, the grep command will process a large number of files in a given command. For instance, grep sometext * will examine every filename in the current directory for “sometext”. However, there is a limit to the number of files that can be handled in a single command. If you ask grep to process too many files, it will produce an error saying “Too Many Files” or the equivalent (depending on your shell).

A tool called xargs can get around this limitation. To invoke it, however, requires some circumspection. For instance, to search every file on a system for “ABCDEFGH”, you would use the following command:

find / -print | xargs grep 'ABCDEFGH'

This will search every file on a machine for the string “ABCDEFGH”, but will not run into the typical errors that result when too many files are open. Usually this limit is a function of the kernel that allows only so many pages of memory to be devoted to command-line arguments. Short of recompiling the kernel for a larger value, using xargs is your best bet.

Matching strings across multiple lines

At the start of this book, we said that grep cannot match strings if they span multiple lines, but that isn’t precisely true. Although most versions of grep cannot handle multiple lines easily, grep -P can overcome this in multiline mode. For instance, take the following file:

red
dog

Normal grep tricks, even specifying the newline character, will not match if you want to search for a line with “red” and the following line “dog”.

$ grep -E 'red\ndog' test
$ grep -G 'red\ndog' test
$ grep -F 'red\ndog' test

However, this is possible if you use PCRE_MULTILINE with grep -P:

$ grep -P '(?m)red\ndog' test
red
dog

This allows a user to overcome the limitation in grep where it will examine only individual lines. This is also one of the many reasons why grep -P tends to be used for more powerful searching applications.

Finally, there are many websites and forums out there where you can find useful regular expression patterns for particular applications. Odds are that others have used regular expressions and grep to pull content out of that same application. The possibilities are endless.

References

re_format(7) manpage

regex(3) manpage

grep(1) manpage

pcre(3) manpage

pcrepattern(3) manpage

Friedl, Jeffery E.F. (2006). Mastering Regular Expressions. O’Reilly Media, Inc.

Index

A note on the digital index

A link in an index entry is displayed as the section title in which that entry appears. Because some sections have multiple index markers, it is not unusual for an entry to have several links to the same section. Clicking on any link will take you directly to the place in the text in which the marker appears.

Symbols

" (double quotes), using regular expressions and, Quotation Marks and Regular Expressions

$ (dollar sign), end of line character, Metacharacters, Metacharacters

' (single quotes), using regular expressions, Quotation Marks and Regular Expressions

( ) (parentheses), Metacharacters, Crafting a Regular Expression, Extended Regular Expressions (egrep or grep -E)

extended regular expressions, Extended Regular Expressions (egrep or grep -E)

precedence in regular expressions, Crafting a Regular Expression

subpatterns, Metacharacters

( ) parentheses, Metacharacters

* (asterisk) quantifier, Introduction to Regular Expressions, Metacharacters, Metacharacters

+ (plus) quantifier, Metacharacters, Metacharacters, Extended Regular Expressions (egrep or grep -E)

extended regular expressions and, Extended Regular Expressions (egrep or grep -E)

- (dashes), indicating ranges, Metacharacters, Metacharacters

. (dot), Metacharacters, Metacharacters

> (redirects), grep Basics

? (question mark) quantifier, Metacharacters, Metacharacters, Extended Regular Expressions (egrep or grep -E)

optional expressions, Extended Regular Expressions (egrep or grep -E)

[...] (character classes), Metacharacters, Metacharacters

[^...] (negated character class), Metacharacters, Metacharacters

\ (backslashes), Metacharacters, Metacharacters, Extended Regular Expressions (egrep or grep -E)

\ escape characters, Metacharacters, Metacharacters, Metacharacters

\' (end of buffer), Metacharacters, Metacharacters

\< (backslash less-than), Metacharacters

\< (start of word), Metacharacters

\> (backslash greater-than), Metacharacters

\> (end of word), Metacharacters

\` (start of buffer), Metacharacters, Metacharacters

^ (caret), as a start of line, Metacharacters, Metacharacters, Metacharacters

` (backticks), Quotation Marks and Regular Expressions

{min,max} quantifier, Metacharacters

| (pipe), Metacharacters, Metacharacters, grep Basics, grep Basics, Extended Regular Expressions (egrep or grep -E)

chaining commands and, grep Basics

extended regular expressions, Extended Regular Expressions (egrep or grep -E)

standard output, grepping, grep Basics

A

\a (alarm) PCRE escape, Character Types

\A character, Character Types

-A flag, Context Line Control

-a flag, File and Directory Selection

--after-context option, Context Line Control

agrep, Introduction

AIX (Solaris), Introduction

alarm (\a) PCRE escape, Character Types

[:alnum:] POSIX definition, POSIX Character Classes

[:alpha:] POSIX definition, POSIX Character Classes

ASCII character set, Binary File Searching

asterisk (*) quantifier, Introduction to Regular Expressions, Metacharacters, Metacharacters

awk command, Introduction to Regular Expressions

B

\b (word boundary), Metacharacters, Metacharacters, Character Types

\B character, Character Types

-b flag, Output Line Prefix Control, Fixed Strings (fgrep or grep -F)

fgrep, Fixed Strings (fgrep or grep -F)

-B flag, Context Line Control

backreferences, Metacharacters, Metacharacters, Backreferences

backslash greater-than (\>), Metacharacters

backslash less-than (\<), Metacharacters

backslashes (\), Metacharacters, Metacharacters, Extended Regular Expressions (egrep or grep -E)

backticks (`), Quotation Marks and Regular Expressions

--before-context option, Context Line Control

binary files, Conceptual Overview, Binary File Searching

--binary option, Other Options

--binary-files option, File and Directory Selection

[:blank:] POSIX definition, POSIX Character Classes

--byte-offset option, Output Line Prefix Control

C

-C flag, Context Line Control

-c flag (fgrep), Fixed Strings (fgrep or grep -F)

caret (^), as a start of line, Metacharacters, Metacharacters, Metacharacters

cat command, grepping standard output, grep Basics

“chaining” commands, grep Basics

character classes ([...]), Metacharacters, Metacharacters

[:cntrl:] POSIX definition, POSIX Character Classes

--color option, General Output Control, Introduction to grep-Relevant Environment Variables

options, Introduction to grep-Relevant Environment Variables

--colour option, General Output Control

command output, Conceptual Overview

concatenation, writing regular expressions and, Crafting a Regular Expression

confidential material, Copyright-protected or confidential material, Copyright-protected or confidential material

context line control, Context Line Control

--context option, Context Line Control

copyright-protected material, Copyright-protected or confidential material

credit card numbers, Credit card numbers

Ctrl-C, breaking quotes, Crafting a Regular Expression

Ctrl-D, exiting grep, grep Basics

\cX PCRE escape, Character Types

Cygwin, Introduction

D

\d (decimal digit) character, Character Types

\D (non-decimal) character, Character Types

\d escape sequence, grep Basics

-D flag, File and Directory Selection

-d flag, File and Directory Selection

dashes (-), indicating ranges, Metacharacters, Metacharacters

decimal digit (\d) character, Character Types

--devices option, File and Directory Selection

[:digit:] POSIX definition, POSIX Character Classes

--directories option, File and Directory Selection

--disable-perl-regexp configuration, Perl-Style Regular Expressions (grep -P)

dollar sign ($), end of line character, Metacharacters, Metacharacters

dot (.), Metacharacters, Metacharacters

double quotes ("), using regular expressions and, Quotation Marks and Regular Expressions

E

-e flag, Match Control, Fixed Strings (fgrep or grep -F)

fgrep, Fixed Strings (fgrep or grep -F)

-E option, Extended Regular Expressions (egrep or grep -E), Fixed Strings (fgrep or grep -F), When to Use grep -E

\e PCRE escape, Character Types

egrep, Extended Regular Expressions (egrep or grep -E), Fixed Strings (fgrep or grep -F), Extended Regular Expressions (egrep or grep -E)

email addresses, Email addresses

--enable-unicode-properties configuration option, Character Properties

end of buffer (\'), Metacharacters, Metacharacters

end of line ($) character, Metacharacters, Metacharacters

end of word (\>), Metacharacters

env command, Introduction to grep-Relevant Environment Variables

environment variables, Introduction to grep-Relevant Environment Variables, Choosing Between grep Types and Performance Considerations

escape characters, Metacharacters, Metacharacters, Metacharacters

escape sequences (regular expressions), Introduction to Regular Expressions

--exclude option, File and Directory Selection

--exclude-dir option, File and Directory Selection

--exclude-from option, File and Directory Selection

extended regular expressions, Extended Regular Expressions (egrep or grep -E), Fixed Strings (fgrep or grep -F)

F

\f (form feed) PCRE escape, Character Types

-f flag, Match Control, Fixed Strings (fgrep or grep -F)

fgrep, Fixed Strings (fgrep or grep -F)

-F option, Fixed Strings (fgrep or grep -F), Perl-Style Regular Expressions (grep -P), When to Use grep -F, Performance Implications

performance implications and, Performance Implications

Fedora operating system, Perl-Style Regular Expressions (grep -P)

--file option, Match Control

files, searching, Conceptual Overview

--files-with-matches option, General Output Control

--files-without-match option, General Output Control

flags, Basic Regular Expressions (grep or grep -G), Extended Regular Expressions (egrep or grep -E), General Output Control, Output Line Prefix Control

general output control, General Output Control, Output Line Prefix Control

form feed (\f) PCRE escape, Character Types

Free Software Foundation, Introduction

G

\G character, Character Types

-G option, Basic Regular Expressions (grep or grep -G), Extended Regular Expressions (egrep or grep -E), Choosing Between grep Types and Performance Considerations

general output control, General Output Control, Output Line Prefix Control

GLOB wildcards, Metacharacters

“Global Regular-Expression Print”, Introduction

GNU, Introduction, Introduction to Regular Expressions, When to Use grep -P

regular expressions, Introduction to Regular Expressions, When to Use grep -P

[:graph:] POSIX definition, POSIX Character Classes

GREP_COLORS variable, Introduction to grep-Relevant Environment Variables

GREP_OPTIONS variable, Introduction to grep-Relevant Environment Variables, When to Use grep -P

H

-H flag, Output Line Prefix Control

-h flag, Output Line Prefix Control, Fixed Strings (fgrep or grep -F)

fgrep, Fixed Strings (fgrep or grep -F)

hexadecimal strings, Binary File Searching

$HOME environment variable, Quotation Marks and Regular Expressions

HP-UX (Solaris), Introduction

I

i (PCRE_CASELESS), PCRE Options

-i flag, Match Control, Fixed Strings (fgrep or grep -F)

fgrep, Fixed Strings (fgrep or grep -F)

IEEE (Institute of Electrical and Electronics Engineers), POSIX Character Classes

--ignore-case option, Match Control

--include option, File and Directory Selection

--initial-tab option, Output Line Prefix Control

Institute of Electrical and Electronics Engineers (IEEE), POSIX Character Classes

--invert-match option, Match Control

IP addresses, IP addresses

J

Java, using regular expressions, Introduction to Regular Expressions

L

-l flag, General Output Control, File and Directory Selection, Fixed Strings (fgrep or grep -F), Useful Recipes

fgrep, Fixed Strings (fgrep or grep -F)

-L flag, General Output Control

--label option, Output Line Prefix Control

LANG environment variable, Introduction to grep-Relevant Environment Variables

LC_ALL environment variable, Introduction to grep-Relevant Environment Variables

LC_COLLATE environment variable, Introduction to grep-Relevant Environment Variables

LC_CTYPE environment variable, Introduction to grep-Relevant Environment Variables

LC_MESSAGES environment variable, Introduction to grep-Relevant Environment Variables

less command, grep Basics

libpcre program, Perl-Style Regular Expressions (grep -P)

--line-buffered option, Other Options

--line-number option, Output Line Prefix Control

--line-regexp option, Match Control

literals (regular expressions), Introduction to Regular Expressions

[:lower:] POSIX definition, POSIX Character Classes

M

m (PCRE_MULTILINE), PCRE Options

-m flag, General Output Control

MAC addresses, MAC addresses

Mac OS X, Introduction

--max-count option, General Output Control

metacharacters, Introduction to Regular Expressions, Metacharacters, POSIX Character Classes

--mmap option, Other Options

mmap( ) function, Other Options

more command, grep Basics

N

\n (newline) PCRE escape, Character Types

-n flag, Output Line Prefix Control, Fixed Strings (fgrep or grep -F)

fgrep, Fixed Strings (fgrep or grep -F)

{N,} quantifier, Metacharacters, Metacharacters

national language support (NLS), Introduction to grep-Relevant Environment Variables

negated character class ([^...]), Metacharacters, Metacharacters

newline (\n) PCRE escape, Character Types

NLS (national language support), Introduction to grep-Relevant Environment Variables

--no-filename option, Output Line Prefix Control

--no-messages option, General Output Control

non-decimal (\D) character, Character Types

non-whitespace (\S) character, Character Types

non-word character (\W), Metacharacters, Metacharacters, Character Types

--null option, Output Line Prefix Control

--null-data option, Other Options

{N} quantifier, Metacharacters, Metacharacters

O

-o flag, General Output Control

octal searching, Octal Searching

--only-matching option, General Output Control

optional expression (?), Extended Regular Expressions (egrep or grep -E)

“optional” pattern matching, Metacharacters

Oracle, Introduction to Regular Expressions

output control, General Output Control, Output Line Prefix Control

P

-P option, Perl-Style Regular Expressions (grep -P), Introduction to grep-Relevant Environment Variables, When to Use grep -P

parentheses (( )), Metacharacters, Metacharacters, Crafting a Regular Expression, Extended Regular Expressions (egrep or grep -E)

extended regular expressions, Extended Regular Expressions (egrep or grep -E)

precedence in regular expressions, Crafting a Regular Expression

subpatterns, Metacharacters

PCRE (Perl-Compatible Regular Expressions), Perl-Style Regular Expressions (grep -P), Octal Searching, Character Properties, Character Properties, PCRE Options, When to Use grep -P

character properties, Character Properties

octal searching, Octal Searching

options, PCRE Options

when to use, When to Use grep -P

pcre manpage, Perl-Style Regular Expressions (grep -P)

pcrepattern, Character Properties

pcrepattern manpage, Perl-Style Regular Expressions (grep -P)

PCRE_CASELESS (i), PCRE Options

PCRE_DOTALL (s), PCRE Options

PCRE_EXTENDED (x), PCRE Options

PCRE_MULTILINE (m), PCRE Options

Perl-Compatible Regular Expressions (PCRE), Perl-Style Regular Expressions (grep -P), Octal Searching, Character Properties, PCRE Options, When to Use grep -P

character properties, Character Properties

options, PCRE Options

when to use, When to Use grep -P

Perl-style regular expressions, Introduction to Regular Expressions, Perl-Style Regular Expressions (grep -P), Introduction to grep-Relevant Environment Variables, Octal Searching, When to Use grep -P, Backreferences

backreferences and, Backreferences

octal searching, Octal Searching

when to use, When to Use grep -P

phone numbers, U.S.-based phone numbers

pipe (|), Metacharacters, Metacharacters, grep Basics, grep Basics, Extended Regular Expressions (egrep or grep -E)

chaining commands and, grep Basics

extended regular expressions, Extended Regular Expressions (egrep or grep -E)

standard output, grepping, grep Basics

plus (+) quantifier, Metacharacters, Metacharacters, Extended Regular Expressions (egrep or grep -E)

extended regular expressions and, Extended Regular Expressions (egrep or grep -E)

POSIX character classes, POSIX Character Classes

POSIXLY_CORRECT environment variable, Introduction to grep-Relevant Environment Variables

[:print:] POSIX definition, POSIX Character Classes

.profile file, Introduction to grep-Relevant Environment Variables

[:punct:] POSIX definition, POSIX Character Classes

Q

-q flag, General Output Control

question mark (?) quantifier, Metacharacters, Metacharacters, Extended Regular Expressions (egrep or grep -E)

optional expressions, Extended Regular Expressions (egrep or grep -E)

--quiet option, General Output Control

R

\r (return carriage) PCRE escape, Character Types

-R flag, File and Directory Selection

-r flag, File and Directory Selection, Useful Recipes

ranges, matching characters with, Metacharacters, Metacharacters

read( ) function, Other Options

--recursive option, File and Directory Selection

Red Hat operating system, Perl-Style Regular Expressions (grep -P)

redirects (>), grep Basics

regexp, Quotation Marks and Regular Expressions (see regular expressions)

--regexp flag, Match Control

regular expressions, Introduction to Regular Expressions, grep Basics, Quotation Marks and Regular Expressions, Metacharacters, POSIX Character Classes, POSIX Character Classes, Crafting a Regular Expression, grep Basics, Basic Regular Expressions (grep or grep -G), Extended Regular Expressions (egrep or grep -E), Extended Regular Expressions (egrep or grep -E), Fixed Strings (fgrep or grep -F), Perl-Style Regular Expressions (grep -P), Introduction to grep-Relevant Environment Variables

crafting, Crafting a Regular Expression, grep Basics

extended, Extended Regular Expressions (egrep or grep -E), Fixed Strings (fgrep or grep -F)

flags, Basic Regular Expressions (grep or grep -G), Extended Regular Expressions (egrep or grep -E)

metacharacters, Metacharacters, POSIX Character Classes

Perl-style, Perl-Style Regular Expressions (grep -P), Introduction to grep-Relevant Environment Variables

POSIX character classes, POSIX Character Classes

quotation marks, Quotation Marks and Regular Expressions

repetitive match (+) character, Metacharacters, Metacharacters

return carriage (\r) PCRE escape, Character Types

S

\S (non-whitespace) character, Character Types

s (PCRE_DOTALL), PCRE Options

\s (whitespace) character, Character Types

-s flag, General Output Control

sed command, Introduction to Regular Expressions

Sendmail, Introduction to Regular Expressions

--silent option, General Output Control

single quotes ('), using regular expressions and, Quotation Marks and Regular Expressions

Social Security numbers, Social Security numbers

Solaris, Introduction

[:space:] POSIX definition, POSIX Character Classes

“standard input”, grepping, grep Basics

start of buffer (\`), Metacharacters, Metacharacters

start of line (^), Metacharacters, Metacharacters

start of word (\<), Metacharacters

--string option, Fixed Strings (fgrep or grep -F)

strings command, Conceptual Overview

subpatterns, Metacharacters

T

\t (tab) character, Character Types

-T flag, Output Line Prefix Control

tab (\t) character, Character Types

tail command, grep Basics

target strings, Introduction to Regular Expressions

text files, searching, Conceptual Overview

--text option, File and Directory Selection

tput init command, File and Directory Selection

tput reset command, File and Directory Selection

U

-u flag, Output Line Prefix Control

-U option, Other Options

U.S.-based phone numbers, U.S.-based phone numbers

--unix-byte-offsets option, Output Line Prefix Control

[:upper:] POSIX definition, POSIX Character Classes

V

-v flag, Match Control, Fixed Strings (fgrep or grep -F)

fgrep, Fixed Strings (fgrep or grep -F)

-V flag, Other Options

--version option, Other Options

W

\W (non-word character), Metacharacters, Metacharacters, Character Types

\w (word character), Metacharacters, Metacharacters, Character Types

-w flag, Match Control

whitespace (\s) character, Character Types

whoami command, Quotation Marks and Regular Expressions, Crafting a Regular Expression

wildcards, Introduction to Regular Expressions, Metacharacters, Metacharacters, File and Directory Selection

--exclude option, File and Directory Selection

Windows, Introduction

wingrep, Introduction

--with-filename option, Output Line Prefix Control

word boundary (\b), Metacharacters, Metacharacters, Character Types

word character (\w), Metacharacters, Metacharacters, Character Types

--word-regexp option, Match Control

X

x (PCRE_EXTENDED), PCRE Options

\x excape, Binary File Searching

-x flag, Match Control, Fixed Strings (fgrep or grep -F)

fgrep, Fixed Strings (fgrep or grep -F)

[:xdigit:] POSIX definition, POSIX Character Classes

Z

\Z character, Character Types

\z character, Character Types

-Z flag, Output Line Prefix Control

-z flag, Other Options

zipgrep, Introduction

About the Authors

John Bambenek is a programmer and teacher with over ten years of experience. His work at the Internet Storm Center and the University of Illinois, as well as his contributions to SANS information management courses, will help him promote this book and give him a sound base from which to write.

Agnieszka Klus was born in Rzeszow, Poland and came here as a child. She lives in the northwest suburb of Chicago, but considers herself a Chicagoan. She is currently trying to get her Masters in Accountancy at the University of Illinois at Urbana-Champaign, and also works as a system administrator at the Coordinated Science Laboratory.

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