Regex Tester

Regular expressions describe text patterns compactly. This tester runs patterns through your browser's JavaScript engine, so the syntax is exactly what you'd write in a literal /pattern/flags or new RegExp()call — which also means it's slightly different from PCRE, Pythonre, and Go's RE2 flavors.

The flags you'll actually use

• g — global: returns all matches instead of stopping at the first.
• i — case-insensitive.
• m — multiline: ^ and $ match the start and end of each line instead of the whole string.
• s — dotall: . matches newlines too.
• u — unicode: enables Unicode escapes like \p{Letter} and handles surrogate pairs correctly.

Capture groups and named groups

Parentheses (…) create numbered capture groups. Use (?:…) for a non-capturing group when you just need grouping for alternation or repetition. Named groups (?<name>…) are easier to read in code and available on match.groups.

Common gotchas

• Without g, repeated calls to exec()on the same regex return the same first match — don't forget the flag for looping.
• Greedy quantifiers (.+, .*) match as much as possible; add ? (.+?) to make them lazy.
• Catastrophic backtracking can hang the browser — avoid nested unbounded quantifiers like (a+)+ on large inputs.