Compose a regex from named building blocks
Stack literal, class, quantifier, anchor and group blocks to build a pattern visually. Highlights matches live. Runs locally.
\d+/\d+/gHow to build a regex from blocks
Compose an ECMAScript pattern by stacking labelled visual blocks — literals, character classes, quantifiers, anchors, capture groups, and alternation — and watch matches highlight live against your sample text.
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Add visual blocks
Pick from six block types: literal text, character class (\d, \w, \s, or a custom range like [a-z]), quantifier (*, +, ?, {n,m}), anchor (^, $, \b), capture group (capturing, non-capturing, or named), and alternation (|). Each block contributes one segment of the final pattern.
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 · blocks · literalECMA · live · regex[class \d] [quantifier +]loaded · ECMAScriptPCRE · ready - 02
Tune each block
Toggle \d (digits), \w (word chars), \s (whitespace), set a custom character class like [a-zA-Z], or pick a quantifier from the dropdown. Group blocks accept inner-pattern text plus a kind selector for capturing, non-capturing, or named groups. The assembled regex updates synchronously as you click.
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 · mliveconfig · tune · optionsclasses · quantifiers · flags\d+ [a-zA-Z]\w* (?<year>\d{4})flags applied · liveECMA · live - 03
Reorder and verify
Use the ↑ / ↓ buttons to reorder blocks — pattern position carries meaning (anchors first, quantifiers after their target, alternation between siblings). Sample text below highlights every match in real time using the g flag, so feedback is instant.
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-saferun · highlight · outputmatches · live · matchesOrder 12345 ships to zone 7 on 2026-05-18. ───── ── ── ──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 compose visually
Blocks make regex authoring easier to learn and easier to read.
Teach beginners
Show how blocks compose into a regex visually. 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-safeAuthor quickly
Skip syntax memorisation — click your way to a pattern. 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-compliantAudit a pattern
Convert a flat regex into blocks to inspect each piece. 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-gradeBuild with groups
Add named groups via the group block. 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-engineDocument logic
Each block has a label — paste them as comments. 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-readyIterate live
Highlighted sample text gives instant feedback per change. 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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