Technology
Regex Tester & Builder
Test regular expressions with live match highlighting, flag support, captured group display, and a library of common patterns.
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Start typing a pattern and test string to see live results
Highlighted Matches
Matches
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Groups
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Captured Groups
Common Regex Patterns
Regex Quick Reference
Character Classes:
. Any character (except newline)
\d Digit [0-9]
\w Word char [a-zA-Z0-9_]
\s Whitespace
[abc] Any of a, b, or c
[^abc] Not a, b, or c
Quantifiers:
* 0 or more
+ 1 or more
? 0 or 1
{n} Exactly n
{n,m} Between n and m
Anchors:
^ Start of string/line
$ End of string/line
\b Word boundary
Groups:
(...) Capturing group
(?:...) Non-capturing group
(?=...) Positive lookahead
(?!...) Negative lookaheadFAQ
Frequently asked questions about regular expressions
What is a regular expression (regex)?
A regex is a pattern that describes a set of strings. It's used for searching, matching, and manipulating text. For example, \d+ matches one or more digits, and [a-z]+ matches lowercase words.
What do regex flags do?
g (global) finds all matches, not just the first. i (case-insensitive) ignores case. m (multiline) makes ^ and $ match line starts/ends. s (dotAll) makes . match newlines too.
How do I match an email address?
A common pattern is [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}. Note: perfectly validating emails via regex is extremely complex — this covers 99% of valid addresses.
What are capturing groups?
Parentheses () create groups that capture matched text. In 'abc123', the pattern ([a-z]+)(\d+) captures 'abc' in group 1 and '123' in group 2. Use (?:) for non-capturing groups.
What's the difference between greedy and lazy matching?
Greedy (default) matches as much as possible: .* matches the entire string. Lazy (add ?) matches as little as possible: .*? matches the minimum. Example: <.+> vs <.+?> on '<a>text</a>'.
How do I test regex performance?
Avoid catastrophic backtracking by not nesting quantifiers (e.g., (a+)+). Test with long inputs. Use atomic groups or possessive quantifiers when available. Keep patterns simple and specific.