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C# - Grouping Constructs
Grouping constructs delineate sub-expressions of a regular expression and capture substrings of an input string. The following table lists the grouping constructs −
| Grouping construct | Description | Pattern | Matches |
|---|---|---|---|
| ( subexpression ) | Captures the matched subexpression and assigns it a zero-based ordinal number. | (\w)\1 | "ee" in "deep" |
| (?< name >subexpression) | Captures the matched subexpression into a named group. | (?< double>\w)\k< double> | "ee" in "deep" |
| (?< name1 -name2 >subexpression) | Defines a balancing group definition. | (((?'Open'\()[^\(\)]*)+((?'Close-Open'\))[^\(\)]*)+)*(?(Open)(?!))$ | "((1-3)*(3-1))" in "3+2^((1-3)*(3-1))" |
| (?: subexpression) | Defines a noncapturing group. | Write(?:Line)? | "WriteLine" in "Console.WriteLine()" |
| (?imnsx-imnsx:subexpression) | Applies or disables the specified options within subexpression. | A\d{2}(?i:\w+)\b | "A12xl", "A12XL" in "A12xl A12XL a12xl" |
| (?= subexpression) | Zero-width positive lookahead assertion. | \w+(?=\.) | "is", "ran", and "out" in "He is. The dog ran. The sun is out." |
| (?! subexpression) | Zero-width negative lookahead assertion. | \b(?!un)\w+\b | "sure", "used" in "unsure sure unity used" |
| (?< =subexpression) | Zero-width positive lookbehind assertion. | (?< =19)\d{2}\b | "99", "50", "05" in "1851 1999 1950 1905 2003" |
| (?< ! subexpression) | Zero-width negative lookbehind assertion. | (?< !19)\d{2}\b | "51", "03" in "1851 1999 1950 1905 2003" |
| (?> subexpression) | Nonbacktracking (or "greedy") subexpression. | [13579](?>A+B+) | "1ABB", "3ABB", and "5AB" in "1ABB 3ABBC 5AB 5AC" |
csharp_regular_expressions.htm
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