RegEx Tester & Debugger

The "g" flag enables global matching across the entire text instead of stopping at the first match. The "i" flag makes matching case-insensitive. The "m" flag enables multiline mode where ^ and $ match the start/end of each line. The "s" flag allows the dot (.) to match newlines. The "u" flag enables full Unicode code point support, and "y" enables sticky matching at the exact lastIndex position.

A capturing group (pattern) creates a numbered backreference ($1, $2, etc.) that remembers the matched substring for later extraction or replacement. A non-capturing group (?:pattern) groups expressions together for quantifiers or alternations without consuming extra memory or generating backreferences.

Lookarounds are zero-width assertions that check whether a pattern matches without consuming characters or including them in the match result. Positive lookahead (?=abc) asserts that "abc" follows, while negative lookahead (?!abc) asserts that "abc" does not follow. Positive lookbehind (?<=abc) asserts "abc" precedes, and negative lookbehind (?<!abc) asserts "abc" does not precede.

Catastrophic backtracking occurs when nested quantifiers or overlapping alternations (such as (a+)+$) force the regex engine to explore an exponential number of permutations when matching fails against large input. This can cause regular expression denial of service (ReDoS). Our tester includes strict safety loops and execution limits to keep your browser fully responsive.

No. The DevsLoad RegEx Tester & Debugger operates 100% inside your browser’s local JavaScript execution context. Neither your regular expression nor your test strings are uploaded, logged, or sent across the network.