Regex Tester & Evaluator

Quick guide

Type a pattern and sample text; matches highlight live. Every evaluation runs in a background worker with a 250 ms watchdog, so a catastrophic pattern can’t hang the tab.

1 · Write a patternEnter it in the pattern field and toggle flags. Syntax errors surface instantly.

2 · Read the matchesHighlights, capture groups, and exact character offsets appear as you type.

3 · Verify a setSwitch to Test cases to check the pattern against a list of known inputs.

Pattern

matches 0 time 0.0 ms state idle

Highlight matches inside a block of sample text.

Test text

len 0 · caret 0

Replace preview

Tokens: $1–$99 · $<name> · $& whole match · $` before · $' after · $$ literal $.

Matches & capture groups

Pattern library

Saved entries keep pattern, flags, and test text. Export/import is the reliable path — localStorage is unreliable under file:// and is treated as a convenience only.

Click any token to insert it at the cursor in the pattern field.

Flavour differences — this is ECMAScript, not PCRE / Python / .NET

No atomic groups (?>…) or possessive quantifiers a++, a*+.
No recursion (?R) / subroutine calls (?1).
No \A, \Z, \z anchors — use ^, $ with the m flag.
No inline flag groups (?i:…) or mode modifiers mid-pattern.
No conditionals (?(1)yes|no) or branch-reset groups.
\d, \w, \b are ASCII-only; \p{…} Unicode property escapes need the u or v flag.
Lookbehind is supported and variable-length, but engine internals differ from PCRE — verify before shipping cross-engine.
A pattern that passes here can still fail in Python/Java/.NET. When in doubt, test in the target engine too.

Single file · runs from file:// · zero dependencies · all evaluation isolated in a Web Worker.