Regular Expression Tester

Email: /^[a-zA-Z0-9.%+-]+@[a-zA-Z0-9.-]+.[a-zA-Z]{2,}$/. URL: /https?://(www.)?[-a-zA-Z0-9@:%.+~#=]{1,256}.[a-zA-Z0-9()]{1,6}\b/. Phone (US): /^(?\d{3})?[-.\s]?\d{3}[-.\s]?\d{4}$/. Date (YYYY-MM-DD): /^\d{4}-\d{2}-\d{2}$/. IP address: /^(\d{1,3}.){3}\d{1,3}$/. Hex color: /^#?([0-9A-Fa-f]{6}|[0-9A-Fa-f]{3})$/. Username (alphanumeric): /^[a-zA-Z0-9_]{3,16}$/. Password (min 8, 1 upper, 1 lower, 1 digit): /^(?=.[a-z])(?=.[A-Z])(?=.*\d).{8,}$/. This tool includes common patterns library for quick testing.

Flags modify regex behavior, added after closing delimiter (/pattern/flags). Common flags: g (global) - find all matches, not just first. i (ignore case) - case-insensitive matching (A = a). m (multiline) - ^ and $ match line breaks, not just string start/end. s (dotall) - . matches newlines. u (unicode) - proper unicode character handling. y (sticky) - match at exact position. Example: /test/gi finds all "test", "TEST", "Test" occurrences. Without g, stops after first match. Use g for find-all, i for case-insensitive, m for multi-line text. This tool supports all major flags with real-time preview.

Parentheses () create capture groups to extract matched portions. Example: /(\d{3})-(\d{3})-(\d{4})/ in "555-123-4567" captures "555", "123", "4567" as groups 1, 2, 3. Backreferences reuse captured groups: \1, \2 refer to first, second group. Example: /(\w+)\s\1/ matches repeated words ("the the"). Named groups: /(?\d{3})-(?\d{3})/ creates named captures. Non-capturing groups: /(?:abc)+/ groups without capturing (faster). Use cases: swap date formats, find duplicate words, extract structured data. JavaScript: match()[1], replace("$1-$2"). This tool highlights capture groups and shows extracted values.

Quantifiers (* + {n,m}) are greedy by default - match as much as possible. Example: /<.>/ on "text" matches entire string (greedy). Lazy (non-greedy) quantifiers add ? after quantifier - match as little as possible. Example: /<.?>/ matches "", then "" separately. Greedy: .* (zero or more, greedy), .+ (one or more, greedy), .{2,5} (2-5, greedy). Lazy: .*? .+? .{2,5}?. Use lazy for: HTML tag extraction, avoiding over-matching, parsing nested structures. Use greedy for: full line matching, whole word extraction. This tool shows greedy vs lazy behavior with match highlighting.

Basic email regex: /^[^\s@]+@[^\s@]+.[^\s@]+$/ (allows most valid emails). Comprehensive: /^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+.[a-zA-Z]{2,}$/ (stricter). Validation challenges: RFC 5322 spec is complex (allows quoted strings, comments), international domains (unicode characters), new TLDs (.museum, .technology), plus addressing ([email protected]). Best practice: use simple regex for UI validation, send confirmation email for real verification. Don't over-restrict ([email protected] is valid). This tool tests email patterns against real examples and highlights match groups for debugging custom patterns.

Assertions match position, not characters. Positive lookahead (?=pattern) - matches if followed by pattern. Example: /\d(?= dollars)/ matches "5" in "5 dollars" but not "5 euros". Negative lookahead (?!pattern) - matches if NOT followed by pattern. Positive lookbehind (?<=pattern) - matches if preceded by pattern. Example: /(?<=$)\d+/ matches "100" in "$100". Negative lookbehind (?<!pattern). Use cases: password validation (must contain uppercase: (?=.*[A-Z])), extract values after labels, match words except in quotes. Browser support: lookbehind added in ES2018. This tool tests all assertion types with position highlighting.

Common issues: escaping special characters (. * + ? [ ] ( ) { } ^ $ | ) - use backslash . Forgetting anchors - /test/ matches "testing" (use ^test$ for exact match). Wrong flags - case sensitivity (add i flag). Greedy vs lazy - .* vs .*?. Character class mistakes - [a-Z] is wrong (use [a-zA-Z]). Debugging steps: start simple, add complexity incrementally. Test with online tool (this tool!). Use verbose mode if available. Check documentation for your regex flavor (JavaScript vs Python vs PCRE differ). Test edge cases. Use visualization tools. This tool provides live highlighting, match explanation, and error messages for syntax issues.