Regular Expressions Cheatsheet

This site is a reference for Regular Expressions (RegEx)

Last updated on 14 June, 2021 at 09:50:16 Optimized for

Regular Expressions are powerful sequences of characters that define search patterns. Regular Expressions are used in search engines, text processing tools like Sed and Awk, for lexical analysis and a lot more. The concept of Regular Expressions arose around the 1950s and later saw heavy use with the introduction of UNIX text processing tools in the 1980s.

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Syntax and Flavors

There are serveral flavors (different syntax) of RegExp with the two most common being POSIX and PCRE (Perl Compatible Regular Expressions). The IEEE POSIX standard has three sets of compliance which are Basic Regular Expression (BRE), Extended Regular Expressions (ERE) and Simple Regular Expressions (SRE).

Regex Examples

Regular Expressions

Expression Description
^(?:[\t ]*(?:\r?\n|\r))+

Match all empty lines


Syntax Description

Start of string, or start of line in multi-line pattern


Start of string


End of string, or end of line in multi-line pattern


End of string


Word boundary


Not word boundary


Start of word


End of word

Character Classes

Syntax Description

Control character


White space


Not white space




Not digit




Not word


Hexadecimal digit


Octal digit


Syntax Description

Upper case letters


Lower case letters


All letters


Digits and letters




Hexadecimal digits




Space and tab


Blank characters


Control characters


Printed characters


Printed characters and spaces


Digits, letters and underscore


Syntax Description

Lookahead assertion


Negative lookahead


Lookbehind assertion


Negative lookbehind


Negative lookbehind


Once-only Subexpression


Condition [if then]


Condition [if then else]




Syntax Description

0 or more


1 or more


0 or 1


Exactly 3


3 or more


Less than 3


3, 4 or 5

Escape Sequences

^ [ . $ { * ( \ + ) | ? < > are all special characters that need to be escaped if you wish to target (i.e. include) them in your search results
Syntax Description

Escape following character


Begin literal sequence


End literal sequence

Special Characters

Syntax Description

New line


Carriage return




Vertical tab


Form feed


Octal character xxx


Hex character hh

Groups and Ranges

Syntax Description

Any character except new line ( )


A or b


Capture group


Passive (non-capturing) group


Range (a or b or c)


Not (a or b or c)


Lower case letter from a to q


Upper case letter from A to Q


Digit from 0 to 7


Group/subpattern number "x"

Pattern Modifiers

All modifiers (except g) in this table are Perl Compliant Regex (PCRE) modifiers and not POSIX compliant
Syntax Description

Global match




Multiple lines


Treat string as single line


Allow comments and whitespace in pattern


Evaluate replacement


Ungreedy pattern

String Replacement

Syntax Description

Nth non-pa­ssive group


"xyz" in /^(abc(xyz))$/


"xyz" in /^(?:abc)(xyz)$/


Before matched string


After matched string


Last matched string


Entire matched string