Regular Expressions Cheat Sheet

A regular expression (regex or regexp) is a sequence of characters that specifies a search pattern. Usually such patterns are used by string-searching algorithms for "find" or "find and replace" operations on strings, or for input validation.

The below tables assists you on how to use different regular expressions:

Character	Explanation
.	Anything. Any character except newline
a	The character a
ab	The string ab
a|b	a or b
a*	0 or more a's
\	Escapes a special character

Regular Expression Quantifiers

Character	Explanation
*	0 or more
+	1 or more
?	0 or 1
{2}	Exactly 2
{2, 5}	Between 2 and 5
{2,}	2 or more

Regular Expression Groups

Character	Explanation
(...)	Capturing group
(?P<Y>...)	Capturing group named Y
(?:...)	Non-capturing group
(?>...)	Atomic group
(?|...)	Duplicate group numbers
\Y	Match the Y'th captured group
(?P=Y)	Match the named group Y
(?R)	Recurse into entire pattern
(?Y)	Recurse into numbered group Y
(?&Y)	Recurse into named group Y
\g{Y}	Match the named or numbered group Y
\g<Y>	Recurse into named or numbered group Y
(?#...)	Comment

Regular Expression Character Classes

Character	Explanation
[ab-d]	One character of: a, b, c, d
[^ab-d]	One character except: a, b, c, d
[\b]	Backspace character
\d	One digit
\D	One non-digit
\s	One whitespace
\S	One non-whitespace
\w	One word character
\W	One non-word character

Regular Expression Assertions

Character	Explanation
^	Start of string
\A	Start of string, ignores m flag
$	End of string
\Z	End of string, ignores m flag
\b	Word boundary
\B	Non-word boundary
\G	Start of match
(?=...)	Positive lookahead
(?!...)	Negative lookahead
(?<=...)	Positive lookbehind
(?<!...)	Negative lookbehind
(?()|)	Conditional

Regular Expression Flags

Character	Explanation
i	Ignore case
m	^ and $ match start and end of line
s	. matches newline as well
x	Allow spaces and comments
J	Duplicate group names allowed
U	Ungreedy quantifiers
(?iLmsux)	Set flags within regex

Regular Expression Special Characters

Character	Explanation
	Newline
	Carriage return
	Tab
\0	Null character
\YYY	Octal character YYY
\xYY	Hexadecimal character YY
\x{YY}	Hexadecimeal character YY
\cY	Control character Y

Regular Expression Posix Classes

Character	Explanation
[:alnum:]	Letters and digits
[:alpha:]	Letters
[:ascii:]	Ascii codes 0 - 127
[:blank:]	Space or tab only
[:cntrl:]	Control characters
[:digit:]	Decimal digits
[:graph:]	Visible characters, except space
[:lower:]	Lowercase letters
[:print:]	Visible characters
[:punct:]	Visible punctuation characters
[:space:]	Whitespace
[:upper:]	Uppercase letters
[:word:]	Word characters
[:xdigit:]	Hexadecimal digits