Regex Generator
Commercial use OK
380+ models
No watermark
No sign-up needed
Model:
+ GPT-5, Claude, Gemini
Describe what you want to match in plain English and get a flavor-specific regex — PCRE, JavaScript, Python, Go (RE2), or POSIX — with a step-by-step explanation and a live tester showing real matches and misses.
Matches in sample
Explanation + test cases
Advanced options
Result
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Generate regex patterns from natural language with AI.
How to Use Regex Generator
1
Enter your input
Type text, upload a file, or describe what you want. No account needed.
2
Click generate
Our AI processes your request in seconds using the best open-source models.
3
Download & share
Download, copy, or share your result. Free for personal and commercial use.
Use this tool via API
Automate this tool from your own code. OpenAI-compatible REST endpoint, Bearer-token auth, no extra SDK required. Token costs match the web interface.
curl -X POST https://api.free.ai/v1/chat/ \
-H "Authorization: Bearer sk-free-..." \
-H "Content-Type: application/json" \
-d '{"model": "qwen-coder", "messages": [{"role": "user", "content": "Write a Python function that reverses a string."}]}'Regex Generator — FAQ
Describe what you want to match in plain English ("US zip codes", "all URLs except localhost", "balanced parentheses up to depth 3") and get back a precise regex with a flavor-aware syntax (PCRE, JavaScript, Python re, Go regexp, POSIX), step-by-step explanation, and a test panel showing real strings that match vs. don't match.
Regex101 / Regexr / Debuggex are EXPLAINER tools — you write the regex, they explain it. Free.ai is a GENERATOR — you describe what you need in English, it writes the regex. Use Free.ai for the first draft, then paste into Regex101 to test against your live data. Combined workflow saves hours over hand-writing regexes from a cheat sheet.
PCRE (default in PHP, Perl, most modern Linux tools). JavaScript (browser + Node, no lookbehind in Safari ≤16.3). Python re (closest to PCRE, no possessive quantifiers). Go regexp (RE2 — no backreferences, no lookarounds, but linear-time guaranteed safe). POSIX BRE/ERE (grep, sed without -E). Pick the one your target language uses — features like lookbehind / possessive quantifiers / named groups don't port across all flavors.
Honest answer: no regex matches every real-world input perfectly. Email validation alone has a 6,425-character "RFC 5322 compliant" regex nobody actually uses. Free.ai targets the 99% case for the use you describe and warns about known edge cases in the explanation (e.g., "this matches example.co.uk but not example.museum"). Always test against real data before shipping to production.
Yes — paste the regex into the description box prefixed with "explain: " (e.g., `explain: ^(?=.*[A-Z])(?=.*\d).{8,}$`) and the model will break down each token and tell you what it actually matches. Use /code/explain/ for richer code-aware explanation.
Yes — when the chosen flavor supports them. PCRE/Python/JS allow positive/negative lookbehind and lookahead; backreferences work in PCRE/Python/Java but NOT in Go (RE2). The model picks the simplest correct construct and warns when a feature isn't portable.
When asked to match "letters" or "word characters" the model uses Unicode-aware classes (`\p{L}`, `\p{N}` in PCRE/Python/Java; `[\p{L}]` with the `u` flag in JavaScript) instead of legacy ASCII-only `[a-zA-Z]`. Mention "ASCII only" in the prompt if you want the legacy behavior.
The model is instructed to avoid nested quantifiers like `(.+)+` that cause ReDoS. For Go (RE2) the engine guarantees linear time so this is impossible by construction. For PCRE/JS/Python, always test long inputs after — paste into Regex101 with a 10,000-char string and watch the steps counter.
ChatGPT (GPT-4) handles regex well but costs $20/mo and the context isn't regex-tuned. Free.ai's system prompt forces explicit flavor + match/non-match test cases + complexity warnings. Self-hosted Qwen 3 Coder is free under the daily token pool. For unusually weird patterns, premium GPT-5 is available with paid tokens.
Yes — paste a regex and ask "convert this Python regex to Go RE2". The model rewrites it using only the target flavor's supported features and warns when a feature has no equivalent (e.g., backreferences when going Python → Go).
Yes — POST to /v1/chat/ with a regex-specialist system prompt and your description as the user message. Useful for tools that auto-generate regex from user input (no-code form builders, log-parser configurators). See /api/.
Yes — the primary code block contains the raw pattern WITH the language-appropriate delimiters (`/pattern/flags` for JavaScript, `r"pattern"` for Python, `` `pattern` `` for Go). Click Copy and paste directly into your source.
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