Regular expression

Comment

(?#comment)

Everything between (?# and ) is ignored by the regex engine.

Branch reset group

(?|regex)

If the regex inside the branch reset group has multiple alternatives with capturing groups, then the capturing group numbers are the same in all the alternatives.

Atomic group

(?>regex)

Atomic groups prevent the regex engine from backtracking back into the group after a match has been found for the group. If the remainder of the regex fails, the engine may backtrack over the group if a quantifier or alternation makes it optional. But it will not backtrack into the group to try other permutations of the group.

Positive lookahead

(?=regex)

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.

Negative lookahead

(?!regex)

Similar to positive lookahead, except that negative lookahead only succeeds if the regex inside the lookahead fails to match.

Positive lookbehind

(?<=regex)

Matches at a position if the pattern inside the lookbehind can be matched ending at that position.

Negative lookbehind

`(?

Matches at a position if the pattern inside the lookbehind cannot be matched ending at that position.

Lookbehind

(?<=regex|longer regex)

Alternatives inside lookbehind can differ in length.

Lookbehind

(?<=x{n,m})

Quantifiers with a finite maximum number of repetitions can be used inside lookbehind.

Lookbehind

(?<=regex)

The full regular expression syntax can be used inside lookbehind.

Lookbehind

(group)(?<=\1)

Backreferences can be used inside lookbehind. Syntax prohibited in lookbehind is also prohibited in the referenced capturing group.

Keep text out of the regex match

\K

The text matched by the part of the regex to the left of the \K is omitted from the overall regex match. Other than that the regex is matched normally from left to right. Capturing groups to the left of the \K capture as usual.

Lookaround conditional

(?(?=regex)then|else) where (?=regex) is any valid lookaround and then and else are any valid regexes

If the lookaround succeeds, the “then” part must match for the overall regex to match. If the lookaround fails, the “else” part must match for the overall regex to match. The lookaround is zero-length. The “then” and “else” parts consume their matches like normal regexes.

Implicit lookahead conditional

(?(regex)then|else) where regex, then, and else are any valid regexes and regex is not the name of a capturing group

If “regex” is not the name of a capturing group, then it is interpreted as a lookahead as if you had written (?(?=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. The lookaround is zero-length. The “then” and “else” parts consume their matches like normal regexes.

Named conditional

(?(name)then|else) where name is the name of a capturing group and then and else are any valid regexes

If the capturing group with the given name took part in the match attempt thus far, the “then” part must match for the overall regex to match. If the capturing group did not take part in the match thus far, the “else” part must match for the overall regex to match.

Named conditional

(?(<name>)then|else) where name is the name of a capturing group and then and else are any valid regexes

If the capturing group with the given name took part in the match attempt thus far, the “then” part must match for the overall regex to match. If the capturing group did not take part in the match thus far, the “else” part must match for the overall regex to match.

Named conditional

(?('name')then|else) where name is the name of a capturing group and then and else are any valid regexes

If the capturing group with the given name took part in the match attempt thus far, the “then” part must match for the overall regex to match. If the capturing group did not take part in the match thus far, the “else” part must match for the overall regex to match.

Conditional

(?(1)then|else) where 1 is the number of a capturing group and then and else are any valid regexes

If the referenced capturing group took part in the match attempt thus far, the “then” part must match for the overall regex to match. If the capturing group did not take part in the match thus far, the “else” part must match for the overall regex to match.

Relative conditional

(?(-1)then|else) where -1 is a negative integer and then and else are any valid regexes

Conditional that tests the capturing group that can be found by counting as many opening parentheses of named or numbered capturing groups as specified by the number from right to left starting immediately before the conditional. If the referenced capturing group took part in the match attempt thus far, the “then” part must match for the overall regex to match. If the capturing group did not take part in the match thus far, the “else” part must match for the overall regex to match.

Forward conditional

(?(+1)then|else) where +1 is a positive integer and then and else are any valid regexes

Conditional that tests the capturing group that can be found by counting as many opening parentheses of named or numbered capturing groups as specified by the number from left to right starting at the “then” part of conditional. If the referenced capturing group took part in the match attempt thus far, the “then” part must match for the overall regex to match. If the capturing group did not take part in the match thus far, the “else” part must match for the overall regex to match.

Conditional

(?(+1)then|else) where 1 is the number of a capturing group and then and else are any valid regexes

The + is ignored and the number is taken as an absolute reference to a capturing group. If the referenced capturing group took part in the match attempt thus far, the “then” part must match for the overall regex to match. If the capturing group did not take part in the match thus far, the “else” part must match for the overall regex to match.

X(?=Y)

긍정형 전방탐색 (Positive Lookahead)

X가 Y앞에 있을 때

X(?!Y)

부정형 전방탐색 (Negative Lookahead)

X가 Y앞에 없을 때

(?<=Y)X

긍정형 후방탐색 (Positive Lookbehind)

X가 Y뒤에 있을 때

`(?

부정형 후방탐색 (Negative Lookbehind)

X가 Y뒤에 없을 때