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
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
\n
Newline
\r
Carriage return
\t
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