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Regular Expression Advanced Syntax Reference

Grouping and Backreferences
Syntax	Description	Example
(regex)	Round brackets group the regex between them. They capture the text matched by the regex inside them that can be reused in a backreference, and they allow you to apply regex operators to the entire grouped regex.	(abc){3} matches abcabcabc. First group matches abc.
(?:regex)	Non-capturing parentheses group the regex so you can apply regex operators, but do not capture anything and do not create backreferences.	(?:abc){3} matches abcabcabc. No groups.
\1 through \9	Substituted with the text matched between the 1st through 9th pair of capturing parentheses. Some regex flavors allow more than 9 backreferences.	(abc|def)=\1 matches abc=abc or def=def, but not abc=def or def=abc.
Modifiers
Syntax	Description	Example
(?i)	Turn on case insensitivity for the remainder of the regular expression. (Older regex flavors may turn it on for the entire regex.)	te(?i)st matches teST but not TEST.
(?-i)	Turn off case insensitivity for the remainder of the regular expression.	(?i)te(?-i)st matches TEst but not TEST.
(?s)	Turn on "dot matches newline" for the remainder of the regular expression. (Older regex flavors may turn it on for the entire regex.)
(?-s)	Turn off "dot matches newline" for the remainder of the regular expression.
(?m)	Caret and dollar match after and before newlines for the remainder of the regular expression. (Older regex flavors may apply this to the entire regex.)
(?-m)	Caret and dollar only match at the start and end of the string for the remainder of the regular expression.
(?x)	Turn on free-spacing mode to ignore whitespace between regex tokens, and allow # comments.
(?-x)	Turn off free-spacing mode.
(?i-sm)	Turns on the options "i" and "m", and turns off "s" for the remainder of the regular expression. (Older regex flavors may apply this to the entire regex.)
(?i-sm:regex)	Matches the regex inside the span with the options "i" and "m" turned on, and "s" turned off.	(?i:te)st matches TEst but not TEST.
Atomic Grouping and Possessive Quantifiers
Syntax	Description	Example
(?>regex)	Atomic groups prevent the regex engine from backtracking back into the group (forcing the group to discard part of its match) after a match has been found for the group. Backtracking can occur inside the group before it has matched completely, and the engine can backtrack past the entire group, discarding its match entirely. Eliminating needless backtracking provides a speed increase. Atomic grouping is often indispensable when nesting quantifiers to prevent a catastrophic amount of backtracking as the engine needlessly tries pointless permutations of the nested quantifiers.	x(?>\w+)x is more efficient than x\w+x if the second x cannot be matched.
?+, *+, ++ and {m,n}+	Possessive quantifiers are a limited yet syntactically cleaner alternative to atomic grouping. Only available in a few regex flavors. They behave as normal greedy quantifiers, except that they will not give up part of their match for backtracking.	x++ is identical to (?>x+)
Lookaround
Syntax	Description	Example
(?=regex)	Zero-width positive lookahead. Matches at a position where the pattern inside the lookahead can be matched. Matches only the position. It does not consume any characters or expand the match. In a pattern like one(?=two)three, both two and three have to match at the position where the match of one ends.	t(?=s) matches the second t in streets.
(?!regex)	Zero-width negative lookahead. Identical to positive lookahead, except that the overall match will only succeed if the regex inside the lookahead fails to match.	t(?!s) matches the first t in streets.
(?<=regex)	Zero-width positive lookbehind. Matches at a position if the pattern inside the lookahead can be matched ending at that position (i.e. to the left of that position). Depending on the regex flavor you're using, you may not be able to use quantifiers and/or alternation inside lookbehind.	(?<=s)t matches the first t in streets.
(?<!regex)	Zero-width negative lookbehind. Matches at a position if the pattern inside the lookahead cannot be matched ending at that position.	(?<!s)t matches the second t in streets.
Continuing from The Previous Match
Syntax	Description	Example
\G	Matches at the position where the previous match ended, or the position where the current match attempt started (depending on the tool or regex flavor). Matches at the start of the string during the first match attempt.	\G[a-z] first matches a, then matches b and then fails to match in ab_cd.
Conditionals
Syntax	Description	Example
(?(?=regex)then|else)	If the lookahead succeeds, the "then" part must match for the overall regex to match. If the lookahead fails, the "else" part must match for the overall regex to match. Not just positive lookahead, but all four lookarounds can be used. Note that the lookahead is zero-width, so the "then" and "else" parts need to match and consume the part of the text matched by the lookahead as well.	(?(?<=a)b|c) matches the second b and the first c in babxcac
(?(1)then|else)	If the first capturing group took part in the match attempt thus far, the "then" part must match for the overall regex to match. If the first capturing group did not take part in the match, the "else" part must match for the overall regex to match.	(a)?(?(1)b|c) matches ab, the first c and the second c in babxcac
Comments
Syntax	Description	Example
(?#comment)	Everything between (?# and ) is ignored by the regex engine.	a(?#foobar)b matches ab

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