Regex Tester

To test a regex: 1) Enter your pattern in the regex field, 2) Add test text in the input area, 3) See matches highlighted in real-time, 4) View capture groups and match details. Add flags like 'g' (global) and 'i' (case-insensitive) as needed.

Common regex flags: g (global - find all matches), i (case-insensitive), m (multiline - ^ and $ match line boundaries), s (dotall - . matches newlines), u (unicode), y (sticky). Combine flags: /pattern/gi for global case-insensitive matching.

Simple email regex: /^[^\s@]+@[^\s@]+\.[^\s@]+$/. This validates basic format. Full RFC 5322 compliance is extremely complex. For production, use established validation libraries or send a confirmation email instead of complex regex.

* matches zero or more occurrences (optional, any count). + matches one or more occurrences (at least one required). Examples: /a*/ matches '', 'a', 'aaa'; /a+/ matches 'a', 'aaa' but not ''. Use * for optional repetition, + for required.

Capture groups () save matched content for later use. Access in JavaScript: match[1], match[2], etc. Named groups: (?<name>pattern) accessed as match.groups.name. Non-capturing groups (?:pattern) group without capturing. Use for extraction and replacement.

Greedy (default) matches as much as possible: /.+/ matches entire string. Lazy (add ?) matches as little as possible: /.+?/ matches minimal content. Example: /<.+>/ on '<a><b>' matches '<a><b>'; /<.+?>/ matches '<a>' then '<b>'.

Escape with backslash: \. \* \+ \? \[ \] \( \) \{ \} \^ \$ \| \\. In JavaScript strings, double-escape: '\\.' for literal dot. Or use new RegExp(string.replace(/[.*+?^${}()|[\]\\]/g, '\\$&')) to escape user input.

Lookahead (?=pattern) matches if followed by pattern, (?!pattern) if not. Lookbehind (?<=pattern) matches if preceded by pattern, (?<!pattern) if not. They don't consume characters. Example: /\d+(?=px)/ matches '10' in '10px' but not in '10em'.

Use \b for word boundaries (between \w and \W characters). /\bcat\b/ matches 'cat' but not 'category' or 'bobcat'. \B matches non-boundaries. Word boundaries are essential for matching whole words in text.

Slow regex causes: 1) Catastrophic backtracking from nested quantifiers like (a+)+, 2) Overly broad patterns with .* 3) Excessive alternation. Solutions: use atomic groups, possessive quantifiers, or specific character classes instead of dot-star.