Regex Tester

Write a pattern, pick flags, and see every match highlighted in your test string as you type. Each match is listed with its index, numbered capture groups, and named groups from (?<name>…) syntax. Invalid patterns show the exact engine error instead of failing silently.

Replace mode previews the result of .replace() with a template, including $1, $<name>, and $& references — handy for checking a substitution before it goes into code.

Common uses

• Debug why a pattern matches too much or too little
• Extract capture groups from sample log lines
• Preview a search-and-replace before running it across a codebase

Example

pattern (?<year>\d{4}) against "born 1815"

1 match · year: 1815

Limitations

• Uses your browser's JavaScript regex engine — PCRE-only syntax (possessive quantifiers, recursion) won't work
• Without the g flag only the first match is reported, mirroring JavaScript semantics

Why does my regex only find the first match?

You are missing the g (global) flag. Without it JavaScript stops after the first match, and this tool mirrors that behavior exactly so results match what your code will do — add g to the flags field to find every occurrence.

Is the text I test my regex against sent anywhere?

No. Matching runs in your browser's own JavaScript engine and nothing is uploaded, so pasting production log lines or user data to debug a pattern is safe.

Does this tester support PCRE features like possessive quantifiers?

No. It uses the JavaScript regex engine, so PCRE-only syntax such as possessive quantifiers (a++), atomic groups, and recursion throws an error. Lookahead, lookbehind, named groups, and Unicode property escapes all work in modern browsers.

How do I use a capture group in the replacement text?

Reference numbered groups as $1, $2, and so on, named groups as $<name>, and the whole match as $&. Replace mode previews the exact output of JavaScript's .replace() with your template before you commit it to code.

What do the regex flags g, i, and m mean?

g finds all matches instead of stopping at the first, i makes matching case-insensitive, and m makes ^ and $ match at each line break rather than only the start and end of the whole string. JavaScript also supports s (dot matches newlines), u (Unicode mode), and y (sticky).