*

Specifies zero or more matches; for example, \w* or (abc)*. Equivalent to {0,}.

+

Specifies one or more matches; for example, \w+ or (abc)+. Equivalent to {1,}.

?

Specifies zero or one matches; for example, \w? or (abc)?. Equivalent to {0,1}.

{n}

Specifies exactly n matches; for example, (pizza){2}.

{n,}

Specifies at least n matches; for example, (abc){2,}.

{n,m}

Specifies at least n, but no more than m, matches.

*?

Specifies the first match that consumes as few repeats as possible (equivalent to lazy *).

+?

Specifies as few repeats as possible, but at least one (equivalent to lazy +).

??

Specifies zero repeats if possible, or one (lazy ?).

{n}?

Equivalent to {n} (lazy {n}).

{n,}?

Specifies as few repeats as possible, but at least n (lazy {n,}).

{n,m}?

Specifies as few repeats as possible between n and m (lazy {n,m}).

(subexpression)

Captures the matched subexpression (or noncapturing group; for more information, see the ExplicitCapture option in Regular Expression Options). Captures using () are numbered automatically based on the order of the opening parenthesis, starting from one. The first capture, capture element number zero, is the text matched by the whole regular expression pattern.

(?<name>subexpression)

Captures the matched subexpression into a group name or number name. The string used for name must not contain any punctuation and cannot begin with a number. You can use single quotes instead of angle brackets; for example, (?'name').

(?<name1-name2>subexpression)

(Balancing group definition.) Deletes the definition of the previously defined group name2 and stores in group name1 the interval between the previously defined name2 group and the current group. If no group name2 is defined, the match backtracks. Because deleting the last definition of name2 reveals the previous definition of name2, this construct allows the stack of captures for group name2 to be used as a counter for keeping track of nested constructs such as parentheses. In this construct, name1 is optional. You can use single quotes instead of angle brackets; for example, (?'name1-name2').

(?:subexpression)

(Noncapturing group.) Does not capture the substring matched by the subexpression.

(?imnsx-imnsx:subexpression)

Applies or disables the specified options within the subexpression. For example, (?i-s: ) turns on case insensitivity and disables single-line mode. For more information, see Regular Expression Options.

(?=subexpression)

(Zero-width positive lookahead assertion.) Continues match only if the subexpression matches at this position on the right. For example, \w+(?=\d) matches a word followed by a digit, without matching the digit. This construct does not backtrack.

(?!subexpression)

(Zero-width negative lookahead assertion.) Continues match only if the subexpression does not match at this position on the right. For example, \b(?!un)\w+\b matches words that do not begin with un.

(?<=subexpression)

(Zero-width positive lookbehind assertion.) Continues match only if the subexpression matches at this position on the left. For example, (?<=19)99 matches instances of 99 that follow 19. This construct does not backtrack.

(?<!subexpression)

(Zero-width negative lookbehind assertion.) Continues match only if the subexpression does not match at the position on the left.

(?>subexpression)

(Nonbacktracking subexpression (also known as a "greedy" subexpression.)) The subexpression is fully matched once, and then does not participate piecemeal in backtracking. (That is, the subexpression matches only strings that would be matched by the subexpression alone.)

By default, if a match does not succeed, backtracking searches for other possible matches. If you know backtracking cannot succeed, you can use a nonbacktracking subexpression to prevent unnecessary searching, which improves performance.

0

0 (default name)

((?<One>abc)\d+)?(?<Two>xyz)(.*)

1

1 (default name)

((?<One>abc)\d+)

2

2 (default name)

(.*)

3

One

(?<One>abc)

4

Two

(?<Two>xyz)

\number

Backreference. For example, (\w)\1 finds doubled word characters.

\k<name>

Named backreference. For example, (?<char>\w)\k<char> finds doubled word characters. The expression (?<43>\w)\43 does the same. You can use single quotes instead of angle brackets; for example, \k'char'.