Run a regex replace with live preview
Type a pattern, choose flags, write a replacement template. See before/after side by side with a live replacement count. Runs locally.
Contact alice at example or bob at test for more info.
How to run a regex replace
Pick an ECMAScript pattern, write a replacement template that can pull in $1 backreferences and named groups, and watch the result update live alongside a running replacement count.
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Type a pattern
Drop in a regex. Capture groups (parenthesised sub-patterns) become accessible as $1, $2, $3 in the replacement template; named groups via (?<name>...) become available as $<name>. Toggle g for global replace, i for case-insensitive, m for multiline anchoring, s for dotall, and u for Unicode-aware matching.
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 · pattern · literalECMA · groups · flags · regex/(\w+)@(\w+)/g └─$1─┘ └─$2─┘loaded · ECMAScriptPCRE · ready - 02
Write a replacement template
Mix literal text with replacement tokens: $1..$9 expand numbered capture groups, $& is the entire match, $<name> pulls a named group. Backslash escapes are not interpreted — write literal characters directly. The template applies once per match when g is on, otherwise to the first match only.
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 · template · options$1 · $& · $<name> · flags$1 at $2 → named: $<user> at $<host>flags applied · liveECMA · live - 03
Read the result
Before and after sit side by side with a live replacement count in the stats bar. Tweak the pattern, flags, or template and the right-hand pane re-renders synchronously; the count surfaces how many sites the rule actually touched so you can spot over- or under-matching at a glance.
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 · after · outputdiff · count · matchesalice at example or bob at test 2 replacements appliedmatches · 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 regex replace
From quick reformats to migration scripts.
Reformat text
Re-order tokens with capture groups. 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-safeAnonymize data
Strip names, emails, or IDs from logs. 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-compliantClean markup
Strip tags or wrap content in new tags. 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-gradeRefactor code
Apply structural renames across a file. 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-engineBuild templates
Turn matched fields into named placeholders. 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-readyMigration scripts
Mass-rewrite identifiers during a port. 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
Something missing?
Tell us what you need. We ship the top requests every two weeks.