Regex Tester

Basic: \d3\.\d3\.\d3\.\d3. This matches the format but allows invalid IPs like 999.999.999.999. For strict validation, use a library.

ISO 8601: \d4-\d2-\d2T\d2:\d2:\d2. For Apache/Nginx: \d2/\w3/\d4:\d2:\d2:\d2. Capture groups to extract components.

Use the s flag to make . match newlines. Or use [\s\S] instead of .. With m flag, ^ and $ match per line.

By default, .* matches as much as possible (greedy). .*? matches as little as possible (non-greedy/lazy). Important for parsing: ".*?" matches one quoted string, not everything between first and last quote.

(?=...) positive lookahead, (?!...) negative lookahead, (?<=...) positive lookbehind, (?<!...) negative lookbehind. They match a position without consuming characters.

When a regex has ambiguous quantifiers, the engine tries exponentially many paths. (a+)+b on "aaaaaaaaac" causes millions of backtracks. The regex hangs.

Avoid nested quantifiers (a+)+, be specific (\d instead of .*), use atomic groups/possessive quantifiers if available, and anchor your patterns with ^ and $.

For simple string operations (starts with, contains, split by delimiter), use built-in string methods — they're faster and more readable. Use regex for pattern matching.

Test with worst-case inputs: long strings that almost match but don't. If a regex takes >1ms per match, it may have backtracking issues. This tool shows match time.

Warden can validate response bodies against patterns — ensuring your API returns expected content, not error pages. Regex-based content checks catch silent failures.