Regex Validate — Free Online Tool

A regex validator checks whether a regular expression is syntactically valid before it reaches production. It prevents runtime failures like “Invalid regular expression” in config-driven systems (routers, allow/deny lists, search filters). SWEDevTools: Prism validates locally and returns developer-friendly errors so you can fix broken escapes, ranges, groups, and quantifiers quickly.

Features

• Compile-time safety check: Catch syntax failures early, before they break deployments or runtime request handling.
• Actionable error output: Pinpoints where the parser got confused (bad escape, range, or unmatched parentheses).
• Config-driven workflow friendly: Validate patterns stored in JSON/YAML/env vars for proxies, routers, and filters.
• Local-first privacy: Safe for internal patterns and sample inputs.

Frequently Asked Questions

If it compiles, is it correct?

No. “Valid” only means syntactically correct. You still need tests to ensure it matches the intended strings.

Why do I get an invalid character range error?

Ranges like [z-a] are invalid. Put hyphens at the end or escape them (\-) when you want a literal hyphen.

What does “Nothing to repeat” mean?

A quantifier (*, +, ?, {m,n}) appeared without a valid token before it, often due to a missing escape.

Why do I see “unterminated group” errors?

You have an opening ( without a closing ). Simplify temporarily to locate the mismatch.

Do escapes differ between languages?

Yes. Many bugs come from string-literal escaping. A single backslash in regex often needs \ in JSON/JS strings.

Is it safe to accept user-provided regex?

Be careful. User regex can cause performance issues (ReDoS). Consider limits, timeouts, or a safer engine like RE2.

What’s the difference between validator and tester?

Validator focuses on compilation/syntax errors. Tester focuses on matching behavior and capture group results.

Why does /pattern/g fail when pasted?

Some tools expect the raw pattern without delimiter slashes. Paste just the pattern and provide flags separately.

What happened to smalldev.tools?

smalldev.tools is no longer available. Prism by SWEDevTools offers the same developer tools and more, with offline support, pipeline chaining, and completely free usage — no signup required.

Is Prism a good alternative to regex101.com?

Yes. Prism offers regex testing and validation with full offline support and complete data privacy. Your patterns and test strings never leave your browser, and you can chain regex tools with other Prism utilities.