Regex Tester

Regex is dense enough that most mistakes are invisible on a read-through. A greedy quantifier that grabs more than intended, a capture group that silently stops participating in one branch of an alternation, a flag that changes how ^ and $ behave, none of these show up as a syntax error. The pattern compiles fine and runs fine. It just matches the wrong thing, or the right thing in the wrong places, and the only reliable way to catch that before it ships is to run it against real sample text and look at exactly what got captured.

That is what this tool is for: type a pattern, paste in text that looks like what you will actually be matching against in production, and see every match highlighted along with its numbered and named capture groups. If the highlighted spans do not line up with what you expected, that gap is the bug, and it is much faster to spot visually than to trace through the pattern character by character.

The flag toggles map directly to JavaScript's actual RegExp flags: g finds every match instead of stopping at the first, i ignores case, m makes ^ and $ match line boundaries instead of only the start and end of the whole string, s lets . match newlines, u switches to Unicode code-point semantics, and y forces the match to start exactly at the current position. Toggling them here and watching the highlighted matches change is a faster way to build an intuition for what each one actually does than reading documentation.