Anchors
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^
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Start of string, or start of line in multi-line pattern
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\A
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Start of string
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$
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End of string, or end of line in multi-line pattern
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\Z
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End of string
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\b
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Word boundary
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\B
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Not word boundary
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\<
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Start of word
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\>
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End of word
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Character Classes
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\c
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Control character
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\s
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White space
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\S
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Not white space
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\d
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Digit
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\D
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Not digit
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\w
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Word
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\W
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Not word
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\x
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Hexadecimal digit
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\O
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Octal digit
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POSIX
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[:upper:]
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Upper case letters
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[:lower:]
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Lower case letters
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[:alpha:]
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All letters
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[:alnum:]
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Digits and letters
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[:digit:]
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Digits
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[:xdigit:]
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Hexadecimal digits
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[:punct:]
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Punctuation
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[:blank:]
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Space and tab
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[:space:]
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Blank characters
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[:cntrl:]
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Control characters
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[:graph:]
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Printed characters
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[:print:]
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Printed characters and spaces
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[:word:]
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Digits, letters and underscore
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Assertions
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?=
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Lookahead assertion
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?!
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Negative lookahead
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?<=
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Lookbehind assertion
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?!= or ?<!
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Negative lookbehind
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?>
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Once-only Subexpression
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?()
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Condition [if then]
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?()|
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Condition [if then else]
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?#
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Comment
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Quantifiers
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*
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0 or more
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{3}
|
Exactly 3
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+
|
1 or more
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{3,}
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3 or more
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?
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0 or 1
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{3,5}
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3, 4 or 5
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Add a ? to a quantifier to make it ungreedy.
Escape Sequences
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\
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Escape following character
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\Q
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Begin literal sequence
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\E
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End literal sequence
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“Escaping” is a way of treating characters which have a special meaning in regular expressions literally, rather than as special characters.
Common Metacharacters
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^
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[
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.
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$
|
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{
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*
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(
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\
|
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+
|
)
|
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?
|
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<
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>
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The escape character is usually \
Special Characters
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\n
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New line
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\r
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Carriage return
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\t
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Tab
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\v
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Vertical tab
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\f
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Form feed
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\xxx
|
Octal character xxx
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\xhh
|
Hex character hh
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|
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Groups and Ranges
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.
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Any character except new line (\n)
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(a|b)
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a or b
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(…)
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Group
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(?:…)
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Passive (non-capturing) group
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[abc]
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Range (a or b or c)
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[^abc]
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Not (a or b or c)
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[a-q]
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Lower case letter from a to q
|
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[A-Q]
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Upper case letter from A to Q
|
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[0-7]
|
Digit from 0 to 7
|
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\x
|
Group/subpattern number “x”
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Pattern Modifiers
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g
|
Global match
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i *
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Case-insensitive
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m *
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Multiple lines
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s *
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Treat string as single line
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x *
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Allow comments and whitespace in pattern
|
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e *
|
Evaluate replacement
|
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U *
|
Ungreedy pattern
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String Replacement
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$n
|
nth non-passive group
|
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$2
|
“xyz” in /^(abc(xyz))$/
|
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$1
|
“xyz” in /^(?:abc)(xyz)$/
|
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$`
|
Before matched string
|
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$’
|
After matched string
|
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$+
|
Last matched string
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$&
|
Entire matched string
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Some regex implementations use \ instead of $.
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