Regex Tester

Regular expressions are powerful but unforgiving — a single misplaced escape, a greedy quantifier where lazy was needed, or a forgotten flag can make a pattern silently match the wrong things. Worse, they fail silently: the regex compiles, runs, and returns no matches when it should have found dozens. A live regex tester gives you instant visual feedback: paste your pattern, paste sample input, and see exactly which substrings match.

Utilify uses the JavaScript (ECMAScript) regex engine — the same one your code runs against in the browser or Node.js. Capture groups, named groups, lookahead, lookbehind, and Unicode property escapes all behave as they would when you call .match() or .exec() in your own code. Toggle the standard flags (g, i, m, s, u) to test variants without re-typing.

The flags change everything, and they are the most common source of confusion. The "g" (global) flag finds every match instead of just the first; "i" makes matching case-insensitive; "m" (multiline) makes "^" and "$" match at line boundaries rather than only at the start and end of the whole string; "s" (dotAll) lets "." match newline characters; and "u" enables full Unicode mode. Flipping these on and off here is the fastest way to understand why a pattern that "should work" is not behaving.

Capture groups are the other half of practical regex work. Parentheses create numbered groups you can pull out of each match, and "(?<name>...)" creates named groups that are far more readable in replacement strings and code. The tester lists every group it captures per match, so you can confirm you are extracting exactly the right slice — the date out of a log line, the domain out of a URL, the version out of a tag — before you wire the pattern into your application.

One practical caution: poorly written patterns with nested quantifiers can trigger catastrophic backtracking, where the engine explores an exponential number of paths and appears to hang. If the page freezes on a particular pattern and input, that is the cause — simplify the pattern or anchor it more tightly, and the problem usually disappears.