What regex flavor does Apache mod_rewrite use?

PCRE. RewriteRule patterns support lookarounds, named groups, and possessive quantifiers — the full PCRE feature set as of the bundled PCRE library version.

What do the [L], [R=NNN], and [QSA] flags mean?

[L] is last — stop processing further rules. [R=NNN] issues an HTTP redirect with the given status code. [QSA] appends the original query string to the rewritten target.

Does the tester actually request the URL?

No. It applies the regex to the URL string and shows the rewritten result. No HTTP request is made.

Is the rule and URL uploaded?

No. Everything runs in your browser.

apache rewriterule · $n captures · flags

Test an Apache .htaccess RewriteRule

Parse a RewriteRule, match it against a URL, and see the rewritten output plus flag actions. Runs locally.

.htaccess Rewrite Tester

Match

yes

rewrites

Captures

groups

Flags

applied

Action

redirect 301

redirect

input · RewriteRule + URL

RewriteRule line

pattern:^old/(.*)$subst:/new/$1flags:L, R=301

Test URL

output · match + rewrite

matched

rewritten → /new/about

action → redirect 301

captures: $1=about

flags: [L] last[R] redirect

Quick Start

How to test an Apache RewriteRule

Paste a RewriteRule line and a URL. See whether the pattern matches, what the substitution produces, and which flags fire.

01
Paste rule
Optional 'RewriteRule' keyword stripped. Pattern, substitution, flags parsed out.
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.
patternECMAScriptliterallive
input · literalrule · regex
^old/(.*)$ /new/$1 [L,R=301]
loaded · ECMAScriptPCRE · ready
02
Paste URL
The URL to match against the pattern (without leading slash typically).
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 · mlive
config · optionsurl · flags
old/about
flags applied · liveECMA · live
03
Read outcome
Match, rewritten URL, captured groups, and flag actions surface together.
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-safe
run · outputresult · matches
ok /new/about redirect 301
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.
copyexportclipboardblob
export · exportedready · saved
Copied
ready to shipsuccess

Pro tips

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.

Use Cases

When to test rewrites

Authoring, auditing, debugging Apache configs.

Debug 404s

Confirm the rule that should rewrite the broken URL. 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-safe

Migrate sites

Verify legacy URL redirects to new paths. 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-compliant

SEO redirects

Sanity-check 301 redirects for canonical paths. 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-grade

Author rules

Iterate on RewriteRule syntax fast. 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-engine

Audit configs

Spot-check what each rule does to representative URLs. 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-ready

Document

Paste outcomes into a runbook. 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-ready

FAQ

Frequently asked questions

Does this support full Apache?

Single-rule semantics only. Multi-rule [L] chaining and RewriteCond aren't modelled — test rules one at a time.

What about query strings?

URL match excludes query string by default in Apache. The QSA flag appends — this tool flags QSA but doesn't synthesise it.

Are $1, $2 supported?

Yes — captured groups expand into the substitution before display.

Flag list?

L, R=301/302, QSA are recognised. Others appear but don't change behaviour here.

Is anything sent to a server?

No. The tool runs entirely in your browser — your pattern and sample text never leave the page.

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