Regex Tester

About this tool

Regex Tester evaluates JavaScript regular expressions against a body of test text and highlights every match in place. The pattern field accepts the full ECMAScript regex syntax — anchors, character classes, alternation, lookahead, lookbehind, named capture groups, Unicode property escapes — with separate toggles for the standard flags (g global, i ignore case, m multiline, s dot-matches-newline, u Unicode, y sticky). Matches are colour-coded and capture groups are listed below the highlight for each one.

Beyond matching, the tool can run a replace preview side-by-side with the original — type the replacement string (with $1 / $<name> backreferences) and see exactly what your regex will produce against the test input. Every operation runs in your browser with the native RegExp engine, so what you debug here is exactly what executes in production JavaScript (Node, Bun, Deno, every browser).

When to use this tool

• Building input validators. Email, phone-number, postcode patterns — design and verify with realistic test data before pushing to code.
• Parsing log files. Iterate a multi-group capture pattern against real log lines until every field comes out clean.
• Bulk editing. Test a find/replace pattern before running it across hundreds of files in your editor.
• Learning regex. Highlighted matches plus instant feedback is the fastest way to internalise look-around, anchors, and backreferences.

About regex flavours

This tool uses JavaScript regex (ECMAScript 2018+). PCRE, Python, .NET, Go, and Java all differ slightly — PCRE supports recursive patterns JavaScript does not, Python's re module lacks lookbehind in some versions, Go uses RE2 which forbids backreferences entirely. If you are targeting a non-JS environment, double-check the flavour-specific features at the receiving end. For pure ECMAScript-compatible patterns this tool is authoritative.