Grep examples: Perl-Compatible Regular Expressions

Last updated: 07 Jan 2024

Table of Contents

All examples run under grep version 3 on Linux (ubuntu)

Match regex

Return the lines containing the given regex in the given file(s)

Example: lines that contain "foobar" followed by a digit:

$ grep -P 'foo\d+' myfile.txt

Matched 2 lines that contain the pattern

Return the lines that fully match the given regex

Use --line-regexp modifier

Example: lines that fully match "foobar" followed by a digit:

$ grep --line-regexp -P 'foo\d+' myfile.txt

Only the line that fully matches the regex is returned

Return lines not containing the given regex.

Use -v modifier

Example: lines that do not contain "foo" followed by digits

$ grep -v -P 'foo\d+' myfile.txt

To match a fixed string (i.e. don't interpret any characters as special characters or modifiers)

Use -F modifier

$ grep -F '()' myfile-special-chars.txt

A simple | between patterns:

Example: lines starting with "foo" OR ending in "bar":

$ grep -P '^foo|bar$' myfile.txt

Simple OR behaviour

Use {N}

Example: lines containing exactly 2 digits:

$ grep -P '\d{2}' myfile.txt

Only lines with exactly 2 digits are a match

Assuming perl-compatible regex (i.e. using the -P modifier)

Class	Description	Equivalent chars
`\w`	Alphanumeric characters plus underscore (_)	`[A-Za-z0-9_]`
`\W`	Everything NOT `\w`	`[^A-Za-z0-9_]`
`\d`	Digits	`[0-9]`
`\s`	Whitespace characters	`[ \t\r\n\v\f]`

To escape any of the special characters .?*+{|()[\^$ use a backslash \:

Example: match a literal '(' character

$ grep -P '\(' myfile.txt

Matched a literal (

Use -- at the end of the pattern and pass the file names:

$ grep -P 'foo.+' -- file1.txt file2.txt

Use -- after the pattern to match multiple files

	Basic Regex	Extended Regex	Perl-compatible Regex
Modifier	No modifier	`-e`	`-P`
Escaping	Special characters lose their special meaning when not escaped	Special characters lose their special meaning when escaped	Special characters lose their special meaning when escaped
Character classes	Supports character classes such as `[[:alnum:]]`	Supports character classes such as `[[:alnum:]]`	Supports character classes such as `\w`, `\d` in addition to classes such as `[[:alnum:]]`

PCRE is not available on BSD grep (such as the MacOS distribution).

Felipe 09 Oct 2022 07 Jan 2024 grep regex