Quick Start
Tutorial
Tools & Languages
Examples
Reference
Book Reviews
RegexBuddy Easily create and understand regular expressions today.
Compose and analyze regex patterns with RegexBuddy's easy-to-grasp regex blocks and intuitive regex tree, instead of or in combination with the traditional regex syntax. Developed by the author of this website, RegexBuddy makes learning and using regular expressions easier than ever. Get your own copy of RegexBuddy now

Use Parentheses for Grouping and Capturing

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.

Only parentheses can be used for grouping. Square brackets define a character class, and curly braces are used by a quantifier with specific limits.

Parentheses Create Numbered Capturing Groups

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.

Using Text Matched By Capturing Groups

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.

Make a Donation

Did this website just save you a trip to the bookstore? Please make a donation to support this site, and you'll get a lifetime of advertisement-free access to this site! Credit cards, PayPal, and Bitcoin gladly accepted.

Regex Tutorial
Introduction
Table of Contents
Special Characters
Non-Printable Characters
Regex Engine Internals
Character Classes
Character Class Subtraction
Character Class Intersection
Shorthand Character Classes
Dot
Anchors
Word Boundaries
Alternation
Optional Items
Repetition
Grouping & Capturing
Backreferences
Backreferences, part 2
Named Groups
Branch Reset Groups
Free-Spacing & Comments
Unicode
Mode Modifiers
Atomic Grouping
Possessive Quantifiers
Lookahead & Lookbehind
Lookaround, part 2
Keep Text out of The Match
Conditionals
Balancing Groups
Recursion
Subroutines
Recursion & Capturing
Recursion & Backreferences
Recursion & Backtracking
POSIX Bracket Expressions
Zero-Length Matches
Continuing Matches
More on This Site
Introduction
Regular Expressions Quick Start
Regular Expressions Tutorial
Replacement Strings Tutorial
Applications and Languages
Regular Expressions Examples
Regular Expressions Reference
Replacement Strings Reference
Book Reviews
Printable PDF
About This Site
RSS Feed & Blog