Regular expressions in grep ( regex ) with examples

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Regular Expressions in grep

How do I use the grep command with regular expressions on a Linux and Unix-like operating systems? How do I use grep and regular expressions (regex)to search for text/ words in Linux?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. Let us see fundamental of regex and how to use regular expressions in the Linux and Unix like systems.

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

[^ ] foo

bar

[0-9]

Three types of regex

The grep understands three different types of regular expression syntax as follows:

basic (BRE) extended (ERE) perl (PCRE)

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 vivek gite:x:1001:1001::/home/vivekgite:/bin/sh gite vivek :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. The following will match word Linux or UNIX in any case:

egrep -i '^(linux|unix)' filename

How to match single characters

The . character (period, or dot) matches any one character. Consider the following demo.txt file:

$ cat demo.txt

Sample outputs:

foo.txt bar.txt foo1.txt bar1.doc foobar.txt foo.doc bar.doc dataset.txt purchase.db purchase1.db purchase2.db purchase3.db purchase.idx foo2.txt bar.txt

Let us find all filenames starting with purchase:

grep 'purchase' demo.txt

Next I need to find all filenames starting with purchase and followed by another character:

grep 'purchase.db' demo.txt

Our final example find all filenames starting with purchase but ending with db:

grep 'purchase..db' demo.txt



How to match only dot (.)

A dot (.) has a special meaning in regex, i.e. match any character. But, what if you need to match dot (.) only? I want to tell my grep command that I want actual dot (.) character and not the regex special meaning of the . (dot) character. You can escape the dot (.) by preceding it with a \ (backslash):

grep 'purchase..' demo.txt

grep 'purchase.\.' demo.txt



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 vivek gite: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

Matching Sets of Characters

How to match sets of character using grep

The dot (.) matches any single character. You can match specific characters and character ranges using [..] syntax. Say you want to Match both ‘Vivek’ or ‘vivek’:

grep '[vV]ivek' filename

OR

grep '[vV][iI][Vv][Ee][kK]' filename

Let us match digits and upper and lower case characters. For example, try to math words such as vivek1, Vivek2 and so on:

grep -w '[vV]ivek[0-9]' filename

In this example match two numeric digits. In other words match foo11, foo12, foo22 and so on, enter:

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.

– Alphanumeric characters. [[:alpha:]] – Alphabetic characters

– Alphabetic characters [[:blank:]] – Blank characters: space and tab.

– Blank characters: space and tab. [[:digit:]] – Digits: ‘0 1 2 3 4 5 6 7 8 9’.

– 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’.

– 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.

– 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

How negates matching in sets

The ^ negates all ranges in a set:

grep '[vV]ivek[^0-9]' test



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 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

Say you just want to match an IP address 192.168.2.254 and nothing else. The following regex to find an IP address 192.168.1.254 will not work (remember the dot matches any single character?):

grep '192.168.1.254' hosts

Sample outputs:

192.168.2.18 centos7 192x168y2z18 centos7

All three dots need to be escaped:

192.168.2.18 centos7

grep '192\.168\.1\.254' hosts

The following example will only match an IP address:

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

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 -E 'word1|word2' filename

### OR ###

egrep 'word1|word2' filename



OR

grep 'word1\|word2' filename

How do I AND with grep?

Use the following syntax to display all lines that contain both ‘word1’ and ‘word2’

grep 'word1' filename | grep 'word2'

OR

grep 'foo.*bar\|word3.*word4' filename

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” words:

egrep 'co{1,2}l' filename

Our next example will match any row of at least three letters ‘c’.

egrep 'c{3,}' filename

In this example, I 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 Highlight with grep?

Pass the --color as follows:

grep --color regex filename

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

Use the following syntax:

grep -o regex filename

grep Regular Expression Operator

I hope following table will help you quickly understand regular expressions in grep when using under Linux or Unix-like systems:

grep regex operator Meaning Example . Matches any single character. grep '.' file

grep 'foo.' input ? The preceding item is optional and will be matched, at most, once. grep 'vivek?' /etc/passwd * The preceding item will be matched zero or more times. grep 'vivek*' /etc/passwd + The preceding item will be matched one or more times. ls /var/log/ | grep -E "^[a-z]+\.log." {N} The preceding item is matched exactly N times. egrep '[0-9]{2} input {N,} The preceding item is matched N or more times. egrep '[0-9]{2,} input {N,M} The preceding item is matched at least N times, but not more than M times. egrep '[0-9]{2,4} input - Represents the range if it’s not first or last in a list or the ending point of a range in a list. grep ':/bin/[a-z]*' /etc/passwd ^ Matches the empty string at the beginning of a line; also represents the characters not in the range of a list. grep '^vivek' /etc/passwd

grep '[^0-9]*' /etc/passwd $ Matches the empty string at the end of a line. grep '^$' /etc/passwd \b Matches the empty string at the edge of a word. vivek '\bvivek' /etc/passwd \B Matches the empty string provided it’s not at the edge of a word. grep '\B/bin/bash /etc/passwd \< Match the empty string at the beginning of word. grep '\ \> Match the empty string at the end of word. grep 'bash\>' /etc/passwd

grep '\ ' /etc/passwd

Linux grep vs egrep command

The egrep is the same as grep -E command. 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.

Conclusion

You learned how to regular expressions (regex) in grep running on Linux or Unix with various examples. See GNU/grep man page online here or see the following resources:

man page grep and regex(7)

info page grep