Regular expression syntax cheat sheet

Assertions

Assertions include boundaries, which indicate the beginnings and endings of lines and words, and other patterns indicating in some way that a match is possible (including look-ahead, look-behind, and conditional expressions).

Boundary-type assertions

Characters	Meaning
^	Matches the beginning of input. If the multiline flag is set to true, also matches immediately after a line break character. For example, /^A/ does not match the "A" in "an A", but does match the first "A" in "An A". Note: This character has a different meaning when it appears at the start of a character class.
$	Matches the end of input. If the multiline flag is set to true, also matches immediately before a line break character. For example, /t$/ does not match the "t" in "eater", but does match it in "eat".
\b	Matches a word boundary. This is the position where a word character is not followed or preceded by another word-character, such as between a letter and a space. Note that a matched word boundary is not included in the match. In other words, the length of a matched word boundary is zero. Examples: /\bm/ matches the "m" in "moon". /oo\b/ does not match the "oo" in "moon", because "oo" is followed by "n" which is a word character. /oon\b/ matches the "oon" in "moon", because "oon" is the end of the string, thus not followed by a word character. /\w\b\w/ will never match anything, because a word character can never be followed by both a non-word and a word character. To match a backspace character ([\b]), see Character Classes.
\B	Matches a non-word boundary. This is a position where the previous and next character are of the same type: Either both must be words, or both must be non-words, for example between two letters or between two spaces. The beginning and end of a string are considered non-words. Same as the matched word boundary, the matched non-word boundary is also not included in the match. For example, /\Bon/ matches "on" in "at noon", and /ye\B/ matches "ye" in "possibly yesterday".

Other assertions

Note: The ? character may also be used as a quantifier.

Characters	Meaning
x(?=y)	Lookahead assertion: Matches "x" only if "x" is followed by "y". For example, /Jack(?=Sprat)/ matches "Jack" only if it is followed by "Sprat". /Jack(?=Sprat|Frost)/ matches "Jack" only if it is followed by "Sprat" or "Frost". However, neither "Sprat" nor "Frost" is part of the match results.
x(?!y)	Negative lookahead assertion: Matches "x" only if "x" is not followed by "y". For example, /\d+(?!\.)/ matches a number only if it is not followed by a decimal point. /\d+(?!\.)/.exec('3.141') matches "141" but not "3".
(?<=y)x	Lookbehind assertion: Matches "x" only if "x" is preceded by "y". For example, /(?<=Jack)Sprat/ matches "Sprat" only if it is preceded by "Jack". /(?<=Jack|Tom)Sprat/ matches "Sprat" only if it is preceded by "Jack" or "Tom". However, neither "Jack" nor "Tom" is part of the match results.
(?<!y)x	Negative lookbehind assertion: Matches "x" only if "x" is not preceded by "y". For example, /(?<!-)\d+/ matches a number only if it is not preceded by a minus sign. /(?<!-)\d+/.exec('3') matches "3". /(?<!-)\d+/.exec('-3') match is not found because the number is preceded by the minus sign.

Groups and backreferences

Groups and backreferences indicate groups of expression characters.

Characters	Meaning
(x)	Capturing group: Matches x and remembers the match. For example, /(foo)/ matches and remembers "foo" in "foo bar". A regular expression may have multiple capturing groups. In results, matches to capturing groups typically in an array whose members are in the same order as the left parentheses in the capturing group. This is usually just the order of the capturing groups themselves. This becomes important when capturing groups are nested. Matches are accessed using the index of the result's elements ([1], …, [n]) or from the predefined RegExp object's properties ($1, …, $9). Capturing groups have a performance penalty. If you don't need the matched substring to be recalled, prefer non-capturing parentheses (see below). String.prototype.match() won't return groups if the /.../g flag is set. However, you can still use String.prototype.matchAll() to get all matches.
(?<Name>x)	Named capturing group: Matches "x" and stores it on the groups property of the returned matches under the name specified by <Name>. The angle brackets (< and >) are required for group name. For example, to extract the United States area code from a phone number, we could use /\((?<area>\d\d\d)\)/. The resulting number would appear under matches.groups.area.
(?:x)	Non-capturing group: Matches "x" but does not remember the match. The matched substring cannot be recalled from the resulting array's elements ([1], …, [n]) or from the predefined RegExp object's properties ($1, …, $9).
\n	Where "n" is a positive integer. A back reference to the last substring matching the n parenthetical in the regular expression (counting left parentheses). For example, /apple(,)\sorange\1/ matches "apple, orange," in "apple, orange, cherry, peach".
\k<Name>	A back reference to the last substring matching the Named capture group specified by <Name>. For example, /(?<title>\w+), yes \k<title>/ matches "Sir, yes Sir" in "Do you copy? Sir, yes Sir!". Note: \k is used literally here to indicate the beginning of a back reference to a Named capture group.

Quantifiers

Quantifiers indicate numbers of characters or expressions to match.

Note: In the following, item refers not only to singular characters, but also includes character classes and groups and backreferences.

Characters	Meaning
x*	Matches the preceding item "x" 0 or more times. For example, /bo*/ matches "boooo" in "A ghost booooed" and "b" in "A bird warbled", but nothing in "A goat grunted".
x+	Matches the preceding item "x" 1 or more times. Equivalent to {1,}. For example, /a+/ matches the "a" in "candy" and all the "a"'s in "caaaaaaandy".
x?	Matches the preceding item "x" 0 or 1 times. For example, /e?le?/ matches the "el" in "angel" and the "le" in "angle." If used immediately after any of the quantifiers *, +, ?, or {}, makes the quantifier non-greedy (matching the minimum number of times), as opposed to the default, which is greedy (matching the maximum number of times).
x{n}	Where "n" is a positive integer, matches exactly "n" occurrences of the preceding item "x". For example, /a{2}/ doesn't match the "a" in "candy", but it matches all of the "a"'s in "caandy", and the first two "a"'s in "caaandy".
x{n,}	Where "n" is a positive integer, matches at least "n" occurrences of the preceding item "x". For example, /a{2,}/ doesn't match the "a" in "candy", but matches all of the a's in "caandy" and in "caaaaaaandy".
x{n,m}	Where "n" is 0 or a positive integer, "m" is a positive integer, and m > n, matches at least "n" and at most "m" occurrences of the preceding item "x". For example, /a{1,3}/ matches nothing in "cndy", the "a" in "candy", the two "a"'s in "caandy", and the first three "a"'s in "caaaaaaandy". Notice that when matching "caaaaaaandy", the match is "aaa", even though the original string had more "a"s in it.
x*? x+? x?? x{n}? x{n,}? x{n,m}?	By default quantifiers like * and + are "greedy", meaning that they try to match as much of the string as possible. The ? character after the quantifier makes the quantifier "non-greedy": meaning that it will stop as soon as it finds a match. For example, given a string like "some <foo> <bar> new </bar> </foo> thing": /<.*>/ will match "<foo> <bar> new </bar> </foo>" /<.*?>/ will match "<foo>"