There are various categories of characters, operators, and constructs that lets you to define regular expressions.
One of them is Grouping Constructs. Grouping constructs describe 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 |
"51", "03" in "1851 1999 1950 1905 2003" |
| (?< ! subexpression) |
Zero-width negative lookbehind assertion. |
(?< !19)\d{2}\b |
"ends", "ender" in "end sends endure lender" |
| (?> subexpression) |
Nonbacktracking (or "greedy") subexpression. |
[13579](?>A+B+) |
"1ABB", "3ABB", and "5AB" in "1ABB 3ABBC 5AB 5AC" |
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