Regular Expressions In grep

How do I use the Grep command with regular expressions under Linux operating systems?

Linux comes with GNU grep, which supports extended regular expressions. GNU grep is the default on all Linux systems. The grep command is used to locate information stored anywhere on your server or workstation.

Regular Expressions

Regular Expressions is nothing but a pattern to match for each input line. A pattern is a sequence of characters. Following all are examples of pattern:

^w1
w1|w2
[^ ]

grep Regular Expressions Examples

Search for 'vivek' in /etc/passswd
grep vivek /etc/passwd
Sample outputs:

vivek:x:1000:1000:Vivek Gite,,,:/home/vivek:/bin/bash
vivekgite:x:1001:1001::/home/vivekgite:/bin/sh
gitevivek:x:1002:1002::/home/gitevivek:/bin/sh

Search vivek in any case (i.e. case insensitive search)
grep -i -w vivek /etc/passwd
Search vivek or raj in any case
grep -E -i -w 'vivek|raj' /etc/passwd
The PATTERN in last example, used as an extended regular expression.

Anchors

You can use ^ and $ to force a regex to match only at the start or end of a line, respectively. The following example displays lines starting with the vivek only:
grep ^vivek /etc/passwd
Sample outputs:

vivek:x:1000:1000:Vivek Gite,,,:/home/vivek:/bin/bash
vivekgite:x:1001:1001::/home/vivekgite:/bin/sh

You can display only lines starting with the word vivek only i.e. do not display vivekgite, vivekg etc:
grep -w ^vivek /etc/passwd
Find lines ending with word foo:
grep 'foo$' filename
Match line only containing foo:
grep '^foo$' filename
You can search for blank lines with the following examples:
grep '^$' filename

Character Class

Match Vivek or vivek:
grep '[vV]ivek' filename
OR
grep '[vV][iI][Vv][Ee][kK]' filename
You can also match digits (i.e match vivek1 or Vivek2 etc):
grep -w '[vV]ivek[0-9]' filename
You can match two numeric digits (i.e. match foo11, foo12 etc):
grep 'foo[0-9][0-9]' filename
You are not limited to digits, you can match at least one letter:
grep '[A-Za-z]' filename
Display all the lines containing either a "w" or "n" character:
grep [wn] filename
Within a bracket expression, the name of a character class enclosed in "[:" and ":]" stands for the list of all characters belonging to that class. Standard character class names are:

• [:alnum:] - Alphanumeric characters.
• [:alpha:] - Alphabetic characters
• [:blank:] - Blank characters: space and tab.
• [:digit:] - Digits: '0 1 2 3 4 5 6 7 8 9'.
• [:lower:] - Lower-case letters: 'a b c d e f g h i j k l m n o p q r s t u v w x y z'.
• [:space:] - Space characters: tab, newline, vertical tab, form feed, carriage return, and space.
• [:upper:] - Upper-case letters: 'A B C D E F G H I J K L M N O P Q R S T U V W X Y Z'.

In this example match all upper case letters:
grep '[:upper:]' filename

Wildcards

You can use the "." for a single character match. In this example match all 3 character word starting with "b" and ending in "t":

grep '\<b.t\>' filename

Where,

• \< Match the empty string at the beginning of word
• \> Match the empty string at the end of word.

Print all lines with exactly two characters:
grep '^..$' filename
Display any lines starting with a dot and digit:
grep '^\.[0-9]' filename

Escaping the dot

The following regex to find an IP address 192.168.1.254 will not work:
grep '192.168.1.254' /etc/hosts
All three dots need to be escaped:
grep '192\.168\.1\.254' /etc/hosts
The following example will only match an IP address:

egrep '[[:digit:]]{1,3}\.[[:digit:]]{1,3}\.[[:digit:]]{1,3}\.[[:digit:]]{1,3}' filename

The following will match word Linux or UNIX in any case:
egrep -i '^(linux|unix)' filename

How Do I Search a Pattern Which Has a Leading - Symbol?

Searches for all lines matching '--test--' using -e option Without -e, grep would attempt to parse '--test--' as a list of options:
grep -e '--test--' filename

How Do I do OR with grep?

Use the following syntax:
grep 'word1|word2' filename

How Do I do AND with grep?

Use the following syntax to display all lines that contain both 'word1' and 'word2'
grep 'word1' filenae | grep 'word2'

How Do I Test Sequence?

You can test how often a character must be repeated in sequence using the following syntax:

{N}
{N,}
{min,max}

Match a character "v" two times:
egrep "v{2}" filename
The following will match both "col" and "cool":
egrep 'co{1,2}l' filename
The following will match any row of at least three letters 'c'.
egrep 'c{3,}' filename
The following example will match mobile number which is in the following format 91-1234567890 (i.e twodigit-tendigit)

grep "[[:digit:]]\{2\}[ -]\?[[:digit:]]\{10\}" filename

How Do I Hightlight with grep?

Use the following syntax:
grep --color regex filename

How Do I Show Only The Matches, Not The Lines?

Use the following syntax:
grep -o regex filename

Regular Expression Operator

grep vs egrep

egrep is the same as grep -E. It interpret PATTERN as an extended regular expression. From the grep man page:

       In basic regular expressions the meta-characters ?, +, {, |, (, and ) lose their special meaning; instead use the backslashed versions \?, \+, \{,
       \|, \(, and \).
       Traditional egrep did not support the { meta-character, and some egrep implementations support \{ instead, so portable scripts should avoid  {  in
       grep -E patterns and should use [{] to match a literal {.
       GNU grep -E attempts to support traditional usage by assuming that { is not special if it would be the start of an invalid interval specification.
       For example, the command grep -E '{1' searches for the two-character string {1 instead of reporting a syntax  error  in  the  regular  expression.
       POSIX.2 allows this behavior as an extension, but portable scripts should avoid it.

References:

• man page grep and regex(7)
• info page grep

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