Step through every regex exec() call
Drive a regex against sample text one exec() at a time. Inspect position, match, groups, and timing per step. Runs locally.
How to debug a regex one step at a time
Drive exec() manually against your sample text and inspect every call individually.
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Paste pattern + flags
Type any regex. The debugger always uses non-global exec so each call is shown discretely.
Patterns compile through the JavaScript RegExp constructor, so syntax errors surface immediately and engine quirks like Unicode property escapes (\p{L}) work when the u flag is on.
patternECMAScriptliteralliveinput · literalpattern · regex(\w+)\s+(\d+)loaded · ECMAScriptPCRE · ready - 02
Add sample text
Drop in the text you want the regex to walk through.
Each flag character mirrors the ECMAScript literal: g enables iteration, i folds case, m anchors per line, s lets . match newlines, u unlocks proper Unicode, y is sticky, and d emits indices.
flagsoptionsg · i · mlivesample · optionstext · flagsalpha 1 beta 22flags applied · liveECMA · live - 03
Inspect each step
For every exec() the debugger reports the call index, position, match, capture groups, and elapsed milliseconds.
Watch for catastrophic backtracking on nested quantifiers — a common ReDoS source. The tool caps match counts to keep the page responsive on pathological patterns.
matchgroupshighlightReDoS-safesteps · outputtrace · matches#1 @0 'alpha 1'matches · livegroups · live - 04
Copy or export
Grab the output as JSON, CSV, or plain text — ready for your editor or pipeline.
Copy writes via navigator.clipboard.writeText; downloads use a Blob and object URL, so nothing transits the network and the output is byte-identical to what you see on screen.
copyexportclipboardblobexport · exportedready · savedCopiedready to shipsuccess
Regex patterns that don't melt down
Use named groups
(?<year>\d{4}) reads better than $1 and survives reordering. Supported in ECMAScript 2018+.
Anchor strictly
Use ^/$ with the m flag when matching whole lines. Without anchors, partial matches sneak through.
Avoid catastrophic backtracking
Nested quantifiers like (a+)+ can explode. Run patterns through ReDoS Detector first.
Escape every literal
Backslash any meta character you want literally: \.\$\^\*\+\?\(\)\[\]\{\}\|\\.
Test edge cases
Empty string, single character, unicode, the maximum length you expect — write fixtures for each.
Patterns aren’t parsers
Don’t parse HTML, JSON, or balanced parens with regex. Use a proper parser when the grammar is recursive.
When to reach for the debugger
Step-by-step insight into matching behaviour.
Track lastIndex
See how position advances after every match. Compiles through the JavaScript RegExp constructor — engine quirks (Unicode property escapes, sticky matching, named groups) work when their flags are on. Error messages cite the exact position of any syntax issue.
ReDoS-safeSpot zero-width loops
Verify your empty-match guard works. Pairs with the ReDoS detector to catch catastrophic backtracking before you deploy. A pattern that handles 1k-character input fine can become a denial-of-service vector on attacker-controlled 1M-character input.
ECMA-compliantBenchmark per-step cost
Find which match dominates total time. Output exports as JavaScript, Python re, Java util.regex, .NET System.Text.RegularExpressions, PCRE, and POSIX — language quirks are highlighted on conversion so you don't ship a JS-only pattern to a Go service.
spec-gradeVerify capture groups
See groups for every match independently. Capture groups (named or numbered) round-trip cleanly into replacement strings, JSON output, or CSV extraction without re-typing. Backreferences ($1, ${name}) preserve semantics across languages.
cross-engineCompare two patterns
Run side-by-side and reason about behaviour. Test against edge cases — empty input, single character, max length, multibyte boundaries — to catch off-by-one and zero-length match bugs before they reach production.
test-readyTeach exec semantics
Walk students through the exec loop. All matching runs client-side so secret patterns never leave the browser. Useful when the pattern itself is sensitive (PII regexes, internal data formats, security filters).
production-readyFrequently asked questions
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Tell us what you need. We ship the top requests every two weeks.