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Regular Expressions

Anchors

^Start of string, or start of line in multi-line pattern
\AStart of string
$End of string, or end of line in multi-line pattern
\ZEnd of string
\bWord boundary
\BNot word boundary
\<Start of word
\>End of word

Character Classes

\cControl character
\sWhite space
\SNot white space
\dDigit
\DNot digit
\wWord
\WNot word
\xHexade­cimal digit
\OOctal digit

POSIX

[:upper:]Upper case letters
[:lower:]Lower case letters
[:alpha:]All letters
[:alnum:]Digits and letters
[:digit:]Digits
[:xdigit:]Hexade­cimal digits
[:punct:]Punctu­ation
[:blank:]Space and tab
[:space:]Blank characters
[:cntrl:]Control characters
[:graph:]Printed characters
[:print:]Printed characters and spaces
[:word:]Digits, letters and underscore

Assertions

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

Quanti­fiers

*0 or more{3}Exactly 3
+1 or more{3,}3 or more
?0 or 1{3,5}3, 4 or 5

Add a ? to a quantifier to make it ungreedy.

Escape Sequences

\Escape following character
\QBegin literal sequence
\EEnd literal sequence

“­Esc­api­ng” is a way of treating characters which have a special meaning in regular expres­sions literally, rather than as special charac­ters.

Common Metach­ara­cters

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

The escape character is usually \

Special Characters

\nNew line
\rCarriage return
\tTab
\vVertical tab
\fForm feed
\xxxOctal character xxx
\xhhHex character hh

Groups and Ranges

.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
\xGroup/­sub­pattern number “­x”

Ranges are inclusive.

Pattern Modifiers

gGlobal 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

String Replac­ement

$nnth 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 $.