Advanced Regex Tester

A capture group (abc) captures the matched substring so you can reference it later — in replace operations as $1 or backreferences as \1. Named groups (?<name>abc) let you reference by name with $<name> or \k<name>. Non-capturing groups (?:abc) group without capturing, useful for alternation or quantifier scope.

Greedy quantifiers (* + ?) match as much as possible. Lazy quantifiers (*? +? ??) match as little as possible. Example: <.+> on '<div><p></p></div>' — greedy matches the whole string, lazy matches just '<div>'. Use lazy when you want the shortest possible match.

Use a lookahead (?=...) when you want to match only if followed by a pattern, without including that pattern in the match. Example: \d+(?= dollars) matches the number in '100 dollars' but not in '100 euros'. Negative lookahead (?!...) does the opposite.

The multiline flag makes ^ and $ match the start and end of each line (separated by \n), instead of only the start and end of the entire string. Without 'm', ^ only matches at position 0. With 'm', it matches after every newline too.

Escape special characters with a backslash: \. for a literal dot, \* for a literal asterisk, \( and \) for literal parentheses. Special regex characters that need escaping: . * + ? ^ $ { } [ ] | ( ) \