Anchors
^
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
Character Classes
\c
Condivol character
\s
White space
\S
Not white space
\d
Digit
\D
Not digit
\w
Word
\W
Not word
\x
Hexadecimal digit
\O
Octal 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
[:cndivl:]
Condivol characters
[:graph:]
Printed characters
[:print:]
Printed characters and spaces
[:word:]
Digits, letters and underscore
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
\Q
Begin literal sequence
\E
End 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
\n
New line
\r
Carriage return
\t
Tab
\v
Vertical tab
\f
Form feed
\xxx
Octal character xxx
\xhh
Hex character hh
Assertions
?=
Lookahead assertion
?!
Negative lookahead
?<=
Lookbehind assertion
?!= or ?<!
Negative lookbehind
?>
Once-only Subexpression
?()
Condition [if then]
?()|
Condition [if then else]
?#
Comment
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
\x
Group/subpattern number "x"
Ranges are inclusive.
Pattern Modifiers
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
u*
UTF-8 (\w)
* PCRE modifier (Perl Compatible Regular Expressions)
string Replacement
$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 $.
