Build a custom username regex
Pick length range, allowed characters, separators, and start-letter rule. Verify against samples live. Runs locally.
/^(?!.*[_]{2})[A-Za-z][A-Za-z0-9_]{2,19}$/How to build a username regex
Pick length range, allowed characters, separators, and whether usernames must start with a letter.
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Set length
Min 3, max 20 is a common default. Adjust to taste.
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 · literallength · regexmin 3 max 20loaded · ECMAScriptPCRE · ready - 02
Pick characters
Letters, digits, underscore, dash — toggle each independently.
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 · optionschars · flags[x] letters digits _flags applied · liveECMA · live - 03
Verify samples
Drop test names. _alice fails (no letter-start).
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 · outputverify · matchesok alice_99 no _alicematches · 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 build a username regex
Enforce a consistent username policy at signup.
Signup forms
Validate username per company policy. 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-safeUsername-like IDs
Reuse for tenant IDs, project slugs. 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-compliantMigration audit
Find existing usernames violating new policy. 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-gradeDocs
Codify policy as a single regex. 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-engineBlock abuse
Forbid consecutive separators to prevent typos. 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
Show how lookaheads enforce no-consecutive. 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.