In short: Regular expressions are write-only code: terrifying to build, useless untested. This tester highlights matches live inside your own text, counts them, and reports invalid patterns with the actual engine error instead of silent failure.
Updated August 28, 2026, after a fresh review, with updated visuals and links.
It runs the JavaScript regex engine entirely in your browser, so patterns and test data never touch a server.
Flags, matches, and the gotchas
Type the pattern without slashes, flip the g/i/m flags, paste test text, and every match lights up yellow in context. The demo pattern \b\w+@\w+\.\w+\b catches simple emails out of a paragraph, the 80% case most people need.
The g flag decides between first-match and all-matches; i makes EMAIL and email equivalent; m lets ^ and $ anchor at line boundaries in multi-line text, which changes results dramatically on lists.
Invalid patterns return the real engine message (like an unclosed group at position 12) so you fix the pattern instead of guessing. Matches cap at 500 to keep page performance sane on giant texts.
Worth remembering
- Build patterns incrementally: start with the smallest piece that matches, then tighten. Full-email-regex in one shot is how people end up copying 6-kilobyte monsters.
- When replacing, remember capture groups are numbered left to right by opening parenthesis; test the group numbers here before wiring $1 into code.
Frequently asked
Do regex flavors differ between languages?
Yes: JavaScript, Python, PCRE and Java differ in lookbehind, Unicode flags and named-group syntax. Most everyday patterns are interchangeable; exotic features should be re-tested in the target language.
Why does my pattern match nothing in the middle of words?
You probably want to remove word boundaries (\b) or add wildcards. \b only matches between a word character and a non-word character, so it cannot fire inside a run of letters.
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