Regular Expression Lab

Test a regular expression against sample text and see every match: its byte and character position, the matched text, and any capture groups (by name where named).

The Regular Expression Lab needs JavaScript to run in your browser. It is client-only. The Regex Reference is entirely static, so its hundred ready-made patterns are readable with no engine at all.

About this lab3 paragraphs

Toggle flags, preview a replacement, read a plain-language explanation of the pattern, get performance notes and generated unit tests, and compare three engines side by side: Rust, your browser's JavaScript, and a PCRE-compatible educational mode with backreferences and lookaround.

The Rust engine runs every pattern in linear time and cannot backtrack catastrophically. The performance notes below describe backtracking engines (the JS and PCRE modes): they are heuristics about shape, not a proof that any pattern is safe.

The Regex Reference lists the same 99 patterns as a static page with no engine to load: the place to look one up, rather than take one apart.

Use it locally This lab has a native command line twin. Build regex-cli from the site's source with:

cargo build --release --bin regex-cli

Source and licence terms