^: The following regular expression must be matched at the beginning of a line, for example, ^this is the start of the line.

$: The preceding regular expression must be matched at the end of a line, for example, this is the end of the line$.

[]: The description of a character class, or a list of characters, is contained within the brackets, and strings that match contain characters in the specified list. Certain character classes can be described using shorthands. We will see some of them throughout the rest of the chapter.

(): This logically groups regular expressions together.

|: This is a logical OR of two regular expressions, for instance, ([expression]) | ([expression]).

?: This matches the preceding regular expression at least once. For example, keith? will match any string that either contains "keith" or doesn't at all.

+: This matches the preceding regular expression at least once.

{n}: This matches the preceding regular expression exactly n times.

{n,m}: This matches the preceding regular expression at least n times and at most m times. For example [0-9]{0,10} will match any decimal number containing between 0 and 10 digits.

[:alnum:]: This matches alphanumeric characters, any decimal digit, or alphabetical character

[:alpha:]: This matches strictly alphabetical characters a-z

[:digit:]: This strictly matches decimal numbers 0-9

[:punt:]: Any punctuation character will be matched

-E or –-extended-regexp: This interprets the PATTERN argument as an extended regular expression

-F or –-fixed-strings: This tells grep to interpret PATTERN as a list of fixed strings separated by newlines to look for in the given file list

For example, the following screenshot shows the output of this command:

-P or –-perl-regexp: This allows grep to interpret PATTERN as a Perl regular expression

-e PATTERN or –-regexp=PATTERN: This forces the PATTERN argument supplied here to be used as PATTERN to match against the input files.

The following command is an example of the usage for the preceding option:

cat /etc/passwd | grep –e '^root' 

The preceding example matches the line that starts with the word root.

-f or –-file=FILE: This grabs a list of patterns to use from the supplied file.

For example, consider a file containing the following text:

^root
^www
^nobody

This file can be used with the –f option as follows:

grep –f patterns.txt < /etc/passwd

-v or –-invert-match: This inverts the matching, which means select or report only file contents that don't match.

-w or –-word-regexp: This report lines from the input files that have whole matching words.

For example, see the output of the following commands:

root@kali:~# grep r –w < /etc/passwd

root@kali:~# grep ro –w < /etc/passwd

root@kali:~# grep root –w < /etc/passwd
root:x:0:0:root:/root:/bin/bash

As you can see from the previous output, and maybe some of your own testing, the first two runs did not describe a complete word of the contents of the /etc/passwd file. However, the last run does; so it's the only one that actually produces output.

-x or –-line-regexp: This reports or prints lines from the input file that have whole lines matching.

-c or –-count: This doesn't report on the matched data, instead prints the number of matches.

-L or –-files-without-match: This prints only the names of files that contain no matches.

-l or –-files-with-matches: This prints only the names of files that contain matches.

-m or –max-count=NUM: This stops processing input after NUM number of matches. If input comes from standard input or using an input redirection, the processing will stop after NUM lines are read.

-o or –only-matching: This prints the matching parts of the input data, each on a separate line.

-a or –-text: This forces binary files to be processed as text. This allows you to operate grep much like the strings utility, which returns all the printable strings from a given file with the added benefit of being able to match the strings using regular expressions.

For example:

grep 'printf' –m 1 –color –text `which echo`

The output of the previous command is as shown in the following screenshot:

--binary-files=TYPE: This checks if a file supplied as input is a binary file. If yes, then it treats the file as the specified TYPE.

-D ACTION or –-devices=ACTION: This processes the input file as a device and uses the ACTION parameter to siphon input from it. By default, ACTION is read.

--exclude=GLOB: This skips any files whose name matches GLOB; wild cards are honored in the matching.

-R, –r, or –-recursive: This processes all the reachable file entries in nested directories from the current directory.