How do I use capture groups and backreferences in regex?

Capture groups are created with parentheses (). Each group captures the matched text and assigns it a number starting from 1. For example, (\d{3})-(\d{4}) matching '555-1234' gives group 1 as '555' and group 2 as '1234'. You can reference these in replacements using $1, $2 etc. Named groups use (? ...) syntax and can be referenced as $ . Non-capturing groups (?:...) group without capturing.

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Regex Visualizer & Tester

Visualize regular expression matches in real time. See highlighted results, capture group details, human-readable pattern explanations, and substitution output — all in your browser. See also the Regex Find & Replace Guide and Regex Builder.

Regular Expression

/ /

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i

m

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Test String

Match Highlighting

Enter a regex pattern above to see highlighted matches.

Pattern Breakdown

Replace Matches

Use $1, $2 for groups. $& for full match.

Result

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Common Pattern Presets

Frequently Asked Questions

What is a regex visualizer and how does it help?▼

A regex visualizer shows you exactly how a regular expression matches text in real time. It highlights matched portions with colored backgrounds, displays capture groups with distinct colors, and breaks the pattern into human-readable explanations. This is much faster than trial-and-error in code. For a deep dive, see our Regex Find & Replace Guide.

What are the most common regex flags and when should I use them?▼

The most common flags are: g (global) to find all matches, i (case-insensitive) to ignore case, m (multiline) so ^ and $ match line boundaries, s (dotall) so dot matches newlines, and u (unicode) for full Unicode support. Use g when you need every occurrence, i for flexible text search, and m for multi-line input.

How do capture groups and backreferences work?▼

Capture groups are created with parentheses (). Each group saves its matched text numbered from 1. For example, (\d{3})-(\d{4}) matching "555-1234" gives group 1 = "555" and group 2 = "1234". In replacements, use $1, $2, etc. Try our Regex Builder to construct patterns visually.

.	Any char (not newline)
\d	Digit [0-9]
\D	Non-digit
\w	Word [a-zA-Z0-9_]
\W	Non-word char
\s	Whitespace
\S	Non-whitespace
[abc]	Any of a, b, c
[^abc]	Not a, b, c
[a-z]	Lowercase letter

*	0 or more
+	1 or more
?	0 or 1
{n}	Exactly n
{n,}	n or more
{n,m}	Between n and m
*?	0+ (lazy)
+?	1+ (lazy)

^	Start of string/line
$	End of string/line
\b	Word boundary
\B	Non-word boundary

(abc)	Capture group
(?:abc)	Non-capturing
(?<n>..)	Named group
\1	Backreference
(?=..)	Lookahead
(?!..)	Neg. lookahead
(?<=..)	Lookbehind
(?<!..)	Neg. lookbehind
a\|b	Alternation

g	Global (all matches)
i	Case-insensitive
m	Multiline ^ $
s	Dotall (. = \n)
u	Unicode

Regex Visualizer & Tester

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Frequently Asked Questions