Regex Cheatsheet

Table of Contents:

Regular Expressions (RegEx) CheatSheet for Developers

Regular Expressions Introduction

Regular expressions (also referred to as rational expressions) are sequences of characters that specify a search pattern in the text. Such patterns are often used in string-searching algorithms to perform “find” and “find and replace” operations on strings, or to validate inputs.

Anchors

Operators Description
^ Start of string, or start of line in multi-line pattern
\A Start of string
$ End of string, or end of line in multi-line pattern
\Z End of string
\b Word boundary
\B Not word boundary
\< Start of word
\> End of word

Quanti­fiers

Operators Occurrence Example Description
? 0 or 1 {3,5} 3, 4 or 5
* 0 or more {3} Exactly 3
+ 1 or more {3,} 3 or more

Add a ? to a quantifier to make it ungreedy.

Groups and Ranges

Operator Description
. Any character except new line (\n)
(a|b) a or b
(...) Group
(?:...) Passive (non-c­apt­uring) group
[abc] Range (a or b or c)
[^abc] Not (a or b or c)
[a-q] Lower case letter from a to q
[A-Q] Upper case letter from A to Q
[0-7] Digit from 0 to 7
\x Group/­sub­pattern number “­x”

Ranges are inclusive.

Escape Sequences

Operator Description
\ Escape following character
\Q Begin literal sequence
\E End literal sequence

Escaping is a way of treating characters which have a special meaning in regex literally, rather than as special charac­ters.

Common Meta-characters

^ [ . $
{ * ( \
+ ) | ?
< >

The escape character is usually \

Special Characters

Operator Description
\n New line
\r Carriage return
\t Tab
\v Vertical tab
\f Form feed
\xxx Octal character xxx
\xhh Hex character hh

Assertions

Operator Description
?= Lookahead assertion
?! Negative lookahead
?<= Lookbehind assertion
?!=
or
?<!		 Negative lookbehind
?> Once-only SubExp­ression
?() Condition [if then]
?()| Condition [if then else]
?# Comment

String Replacement

Operator Description
$n nth non-passive group
$2 “xyz” in /^(abc(xyz))$
$1 “xyz” in /^(?:abc)(xyz)$/
$` Before matched string
$' After matched string
$+ Last matched string
$& Entire matched string

Some regex implementations use \ instead of $.

Pattern Modifiers

Operator Description
g Global match
i * Case-i­nse­nsitive
m * Multiple lines
s * Treat string as single line
x * Allow comments and whitespace in pattern
e * Evaluate replac­ement
U * Ungreedy pattern

* –> PCRE modifier