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Grouping constructs delineate subexpressions of a regular expression and typically capture substrings of an input string. Grouping constructs include the language elements listed in the following table.

PatternDescriptionSampleMatches
( subexpression )Captures the matched subexpression and assigns it a one-based ordinal number.(\w)\1"ee" in "deep"
(?<name> subex)Captures the matched subexpression into a named group.(?<double>\w)\k<double>"ee" in "deep"
(?<name1-name2> subex)Defines a balancing group definition.(((?'Open'\()[^\(\)]*)+((?'Close-Open'\))[^\(\)]*)+)*(?(Open)(?!))$"((1-3)*(3-1))" in "3+2^((1-3)*(3-1))"
(?:subex)Defines a noncapturing group.Write(?:Line)?"WriteLine" in "Console.WriteLine()", "Write" in "Console.Write(value)"
(?imnsx-imnsx: subex)Applies or disables the specified options within subexpression.A\d{2}(?i:\w+)\b"A12xl", "A12XL" in "A12xl A12XL a12xl"
(?= subex)Zero-width positive lookahead assertion.\w+(?=\.)"is", "ran", and "out" in "He is. The dog ran. The sun is out."
(?! subex)Zero-width negative lookahead assertion.\b(?!un)\w+\b"sure", "used" in "unsure sure unity used"
(?<= subex)Zero-width positive lookbehind assertion.(?<=19)\d{2}\b"99", "50", "05" in "1851 1999 1950 1905 2003"
(?<! subex)Zero-width negative lookbehind assertion.(?"51", "03" in "1851 1999 1950 1905 2003"
(?> subex)Nonbacktracking (or "greedy") subexpression.[13579](?>A+B+)"1ABB", "3ABB", and "5AB" in "1ABB 3ABBC 5AB 5AC"