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By placing part of a regular expression inside round brackets or parentheses, you can group that part of the regular expression together. This allows you to apply a quantifier to the entire group or to restrict alternation to part of the regex.
Besides grouping part of a regular expression together, parentheses also create a numbered capturing group. It stores the part of the string matched by the part of the regular expression inside the parentheses.
The regex Set(Value)? matches Set or SetValue. In the first case, the first (and only) capturing group remains empty. In the second case, the first capturing group matches Value.
If you do not need the group to capture its match, you can optimize this regular expression into Set(?:Value)?. The question mark and the colon after the opening parenthesis are the syntax that creates a non-capturing group. The question mark after the opening bracket is unrelated to the question mark at the end of the regex. The final question mark is the quantifier that makes the previous token optional. This quantifier cannot appear after an opening parenthesis, because there is nothing to be made optional at the start of a group. Therefore, there is no ambiguity between the question mark as an operator to make a token optional and the question mark as part of the syntax for non-capturing groups, even though this may be confusing at first. There are other kinds of groups that use the (? syntax in combination with other characters than the colon that are explained later in this tutorial.
color=(?:red|green|blue) is another regex with a non-capturing group. This regex has no quantifiers.
Capturing groups make it easy to extract part of the regex match and reuse the text inside the regular expression (see below), or afterward. What you can do with it afterward depends on the tool or programming language you are using. The most common usage is in search-and-replace operations. The replacement text uses a special syntax to allow text matched by capturing groups to be reinserted. This syntax differs greatly between various tools and languages, far more than the regex syntax does. Please check the replacement text reference for details.
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Page URL: http://www.regular-expressions.info/brackets.html
Page last updated: 09 August 2013
Site last updated: 11 May 2015
Copyright © 2003-2015 Jan Goyvaerts. All rights reserved.
|Table of Contents|
|Regex Engine Internals|
|Character Class Subtraction|
|Character Class Intersection|
|Shorthand Character Classes|
|Grouping & Capturing|
|Backreferences, part 2|
|Branch Reset Groups|
|Free-Spacing & Comments|
|Lookahead & Lookbehind|
|Lookaround, part 2|
|Keep Text out of The Match|
|Recursion & Capturing|
|Recursion & Backreferences|
|Recursion & Backtracking|
|POSIX Bracket Expressions|
|Regular Expressions Quick Start|
|Regular Expressions Tutorial|
|Replacement Strings Tutorial|
|Applications and Languages|
|Regular Expressions Examples|
|Regular Expressions Reference|
|Replacement Strings Reference|
|About This Site|
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