RegexLab

English · Română · Français · Italiano · Español · Português · Deutsch · Magyar

Version: 2.2.0 · Last updated: 2026-10-27

Status: stable · Offline: complete (no network requests)

Interface languages: Română, English, Français, Italiano, Español, Português (PT), Deutsch, Magyar

A regular-expression laboratory: live testing, explanations, replacement, code snippets and a pattern library, in a single HTML file.

Live app: https://chiuta.github.io/RegexLab/

What it is

RegexLab is a tool for writing and testing JavaScript regular expressions right in the browser. The test text never leaves your device. The interface has eight languages (English, Romanian, French, Italian, Spanish, Portuguese, German, Hungarian); the default language is English.

Features

• Debugger (2.0): the Debugger tab runs an instrumented matcher and shows attempts, captures and backtracking step by step (previous/next buttons, slider, autoplay and "Next match"). It is a simplified model of the JavaScript engine (flags i m s, first 400 characters; no lookbehind or \p{…}), used only for explanation. If the matcher exceeds 50,000 steps it stops and reports probable catastrophic backtracking, with rewrite suggestions ((a+)+ → (a+), merged adjacent repetitions, duplicate alternatives; hints for overlaps), the step count before/after on your text and an equivalence check on your samples.
• Pattern from examples (2.0): in Tests, "Generate a pattern…" proposes a pattern from the + lines and checks it against the - lines. In Code, a compatibility table warns about differences from PCRE/PHP, Python and RE2/Go.
• Pattern field (/.../), flags g i m s u v y (u and v are mutually exclusive), validity indicator ("valid" / error), match count and execution time.
• Test string area with match highlighting.
• Tests tab: test cases, one per line — + text must match, - text must not; shows pass/fail per case and the total (evaluated in the same Web Worker, with the same time limit).
• Static (heuristic) warning for nested quantifiers, before a timeout is reached.
• The code snippets adapt named groups for Python/Go ((?P<n>…)) and add notes when the syntax is not available (Go: no lookaround/backreferences; Python: no \p{…}; PHP: \p{…} requires the u modifier).
• Seven tabs: Matches (matches and groups), Explanation (explanation of the pattern), Replace (replacement with $1, $2, $&), Code (ready-to-paste snippets for JavaScript, Python, PHP, Go, Java, C# and Ruby), Library (22 ready-made patterns, in eight languages), Tests (test cases) and Reference (reference sheet).
• Evaluation runs in a Web Worker, with a 1500 ms time limit and a warning message for possible "catastrophic backtracking".
• Copy regex, Copy link (permalink with the pattern, flags, text and replacement in the URL's # fragment) and Reset buttons.

User guide

1. Open the page and choose the language from the language drop-down in the header.
2. Type the pattern in the top field and tick the flags you want.
3. Paste or type the text in Test string; the results appear immediately in the Matches tab.
4. Open Explanation for a breakdown of the pattern.
5. In Replace, type the replacement template ($1, $2, $&) and read the result.
6. In Code, copy the snippet for the language you want with the ⧉ button.
7. In Library, click a pattern to load it; in Reference you will find the basic syntax.
8. Copy link creates a link with the current state; Reset returns to the default example; Esc closes the About window.

Privacy and network

• Local storage (localStorage): seven regexlab-* keys: regexlab-pattern, regexlab-flags, regexlab-subject, regexlab-replace, regexlab-tests, regexlab-lang, regexlab-theme. They stay only in your browser.
• Network: the code contains no network requests, external scripts or fonts. The About window contains links (Buy Me a Coffee, website, contact) that open only on click. No analytics or telemetry.
• The link created with Copy link includes the pattern and the test text in the URL; do not share it if the text is sensitive.

Running locally / offline

Download index.html and open it in a browser; it works without internet.

News in 2.1.0 and what is not implemented (2026-10-27)

• Code generation: besides JavaScript, Python, PHP and Go, the Code tab now generates snippets for Java, C# and Ruby (escaping and flags adapted; notes where the syntax differs). The Java and Ruby snippets were checked by compiling/running them on an example with \d, quotes and flags.
• Library of common expressions: 7 new patterns in the "Developer & web" category (UUID, semantic version SemVer — the official semver.org expression, MAC address, 24 h time, repeated word, Markdown link, leading/trailing whitespace); the library now has 22 patterns. Each was tested with positive and negative samples.
• Other dialects (PCRE, Python, RE2/Go): remain a compatibility table and conversions in the code snippets, not real execution; the engine is the browser's (JavaScript). Really running PCRE/Python/RE2 would need WebAssembly engines (hundreds of KB to MB, against the "no dependencies" principle): planned, not implemented.
• ReDoS alert: already existed (heuristic warning + debugger detecting catastrophic backtracking, 1.5 s limit).

2.2.0: Perl snippet and verification of the generated code (2026-10-27)

• Perl added to the Code tab (while ($text =~ m'…'flags) { print "$&\n"; }; single-quote delimiters so that $ and @ in the pattern are never interpolated; i/m/s mapped, notes for \d \w \s on Unicode strings and for the sticky flag). Texts of the help in 8 languages (fr/it/es/pt/de: machine translation).
• Real-engine verification of the snippets (tests/codegen.test.mjs): for 9 patterns the generated Python, PHP, Go, Java, Ruby and Perl code is run with the real interpreter/compiler installed here and the list of matches is compared with the page's JavaScript engine (117 checks, 52 real executions, all equal). Go (RE2) cases with lookaround/backreference are not run: the test checks that the snippet warns that it will not compile. C# is not run (no .NET here) and a Rust snippet was not added (the regex crate is not available here to verify it).
• This verifies the generated code, not the engines inside RegexLab: PCRE/Python/RE2 are still not executed in the page (that would need WebAssembly engines; against the "no dependencies" principle) — planned, not implemented.

History / Changelog

2.2.0 — 2026-10-27 (previous: 2.1.0)

Perl snippet; test that runs the generated code in real interpreters. PCRE/Python/RE2 execution in the page remains planned.

2.1.0 — 2026-10-27 (previous: 2.0.0)

Java/C#/Ruby code generation; 7 new library patterns; PCRE/Python/RE2 dialects remain a compatibility table (real execution planned).

2.0.0 — 2026-10-26

• Step-by-step visual debugger with anti-ReDoS rewrite suggestions, a pattern generator from +/- examples and compatibility notes (PCRE/PHP, Python, RE2/Go). Tests in tests/debugger.test.mjs.

1.2.9 — 2026-10-23

• Touch targets of at least 44 px on touch devices (pointer:coarse); desktop is unchanged.

1.2.8 — 2026-10-23

• CSP hardened: removed blob: from script-src (the blob: Worker stays allowed via worker-src).

1.2.6 — 2026-10-22

fff-kit 1.1.4: touch targets and minimum text size.

1.2.5 — 2026-10-22

• Length limits for the pattern (10,000 characters), the test text (200,000), the replacement and the test cases, with a translated message: pasting 1 MB no longer freezes the page (matching itself already ran in a Web Worker with a timeout).

1.2.4 — 2026-10-22

• fff-kit 1.1.2: PBKDF2 600,000 iterations, anti-clickjacking protection

1.2.3 — 2026-10-22

• Mobile: small text raised to at least 12 px; the "copy matches" button and the footer link have larger tap targets (the ReDoS timeout in a Web Worker was already in place).

1.2.2 — 2026-10-22

• Field and button labels for screen readers (regular expression, test string, replacement, tests, close, language, theme) are now translated and update when the language changes.

1.2.1 — 2026-10-22

• Translation fixes: the page title and meta tags now follow the interface language; leftover texts in the wrong language removed.

1.1.0 — 2026-10-14

• Backup copy and data labelling (fff-kit).

12 October 2026 — version 1.0.0

• 1.0.0 — 2026-10-12: the version and the date of the last update are shown in the app

11 October 2026 (third pass)

• The v flag (unicodeSets: set operations, e.g. [\p{L}--[a-z]]); u and v are mutually exclusive, and the Python/PHP snippets treat it as u. CSP: hash recalculated.

11 October 2026 (second pass)

• Tests tab with +/- cases; heuristic warning for nested quantifiers; Python/Go/PHP snippets fixed for named groups and with compatibility notes; new texts in all 7 languages. CSP: hash recalculated.

11 October 2026

• Accessibility: the main content is wrapped in <main> (header and footer remain separate landmarks); axe-core reports 0 violations at 1280 and 360 px.
• CSP: script-src uses a SHA-256 hash instead of 'unsafe-inline' (the blob: worker is still allowed); verified with CSP active: matching and the worker work.

Licence

MIT — see the LICENSE file.

Audit

Audit: 2026-10-10 — the README's claims (no network requests, storage only in localStorage) match the code; no fetch/CDN/WebSocket. Contrast and ARIA fixes.

Author

Alexio — Alexandru-Ionuț Chiuță. Contact: [email protected]

Summary

RegexLab is a single-file regular-expression laboratory: live matching with highlighting, explanation, replace, code snippets (JavaScript, Python, PHP, Go, Java, C#, Ruby), a 22-pattern library and a reference sheet, in 8 UI languages. Matching runs in a Web Worker with a 1.5 s timeout. State is kept in localStorage; no network requests or telemetry. MIT licence.