RegexTester.dev is a free, browser-based regex workbench built around real engines: PCRE2, RE2, .NET, Python, Java, Go, Rust and JavaScript run as WebAssembly builds (or natively) so every flavor behaves exactly like it does in production — no approximations, no "JavaScript-flavored" pretending.
What's inside
Regex Debugger — the newest addition. An instrumented backtracking engine records every node attempt, give-back and capture, then lets you scrub the run like a video:
- A zoomable execution flame graph: green = matched, red = failed, blue = gave back; scroll-wheel zoom, drag to pan, click a block to jump
- A Hot spots table ranking every node of your pattern by operations, give-backs, failures and share of total steps
- An input heat map showing which characters were consumed and which were given back during backtracking
- Capture-withdrawal counts (e.g. "263 captures made during this attempt — later backtracking withdrew 261")
- Catastrophic-backtracking detection with dialect-aware rewrites — atomic groups and possessive quantifiers where the flavor supports them — each rewrite equivalence-checked against the real engine before you apply it with one click
Benchmark Suite — compare up to 5 alternative patterns under identical conditions: median/P95/P99 latency, throughput, jitter, sub-clock-precision timing via batch amplification, automatic equivalence checking ("faster because it matches less" gets flagged), and Markdown/JSON export.
Nine flavor dialects — including a Formal Automata Theory flavor for formal-language coursework — plus an AI regex generator whose suggestions are tested across engines for compatibility, and a gallery of 14,000+ copy-ready community patterns.
Everything runs locally in your browser: no account, no cookies, nothing uploaded. Free forever.
