Why the 5000-match cap?

To keep the browser responsive under runaway patterns. Trim the text or refine the pattern if you hit it.

Why is the g flag not matching all occurrences?

Check that `g` is actually in your flags. If the pattern can match an empty string, the tool steps forward automatically to avoid an infinite loop — the same applies in real code.

How do I test named capture groups?

Use `(? ...)`. The tool lists matches by ` ` alongside numbered groups.

Why do PHP or Python produce different matches?

Regex engines differ in Unicode behavior, lookbehind and named-group syntax. These results describe your browser's JavaScript implementation; retest in the target language.

Why can results differ from PHP or Python?

This uses the browser's JavaScript regex engine. Other languages can differ in syntax, Unicode behavior and flags; a match here does not validate every engine.

Regex tester

Live matching, flags, and every capture group listed

Computed in your browser

Enter the pattern without surrounding / delimiters, add flags and supply text. Result offsets start at zero; timed-out calculations are stopped.

Pattern

Flags

Text

Matches

–

Example

Input

Pattern: (cat|dog)
Flags: g
Text: cat and dog

Example result

#0  "cat"
  $1 = "cat"

#8  "dog"
  $1 = "dog"

Offsets start at zero. Do not include surrounding / delimiters in the pattern field.

Technical details & limits

Offsets count JavaScript UTF-16 code units. g finds all matches, i ignores case, m changes line anchors, s lets dot match newlines and u enables Unicode mode. Matching runs in a separate worker with a timeout to limit page blocking from backtracking; output is capped at 5000 matches.

About this tool

Test patterns with the browser's JavaScript regex engine. Inspect match offsets, complete matches and capture groups with g, i, m, s, u and y flags.

Use cases

Sanity-check a regex before dropping it into a log-parsing script.
Debug why an “almost working” regex fails on a specific line.
Pull the values of named capture groups out to inspect structure.

How to use

Enter the expression in Pattern without surrounding / delimiters.
Enter flags in Flags: g (global), i (case-insensitive), m (multiline), s (dot-all), u (Unicode) or y (sticky). Use each at most once.
Paste the target text in Text. Editing any field reruns the match.

Notes

Uses JavaScript’s RegExp engine, which differs in some corners from PCRE / Go / Java (backtracking, atomic groups, conditionals). Ultimately test in your target language.

FAQ

Why the 5000-match cap?: To keep the browser responsive under runaway patterns. Trim the text or refine the pattern if you hit it.
Why is the g flag not matching all occurrences?: Check that `g` is actually in your flags. If the pattern can match an empty string, the tool steps forward automatically to avoid an infinite loop — the same applies in real code.
How do I test named capture groups?: Use `(?<name>...)`. The tool lists matches by `<name>` alongside numbered groups.
Why do PHP or Python produce different matches?: Regex engines differ in Unicode behavior, lookbehind and named-group syntax. These results describe your browser's JavaScript implementation; retest in the target language.
Why can results differ from PHP or Python?: This uses the browser's JavaScript regex engine. Other languages can differ in syntax, Unicode behavior and flags; a match here does not validate every engine.

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