Regex Tester

A regular expression is a sequence of characters that defines a search pattern. Regex is used for pattern matching, text searching, validation, and text replacement operations. It's a powerful tool supported by most programming languages for handling string operations efficiently.

Regex flags modify how the pattern matching works. Common flags include: g (global - find all matches), i (case-insensitive), m (multiline - ^ and $ match line breaks), s (dotall - . matches newlines), and u (unicode). You can combine flags like "gi" for global case-insensitive matching.

A basic email regex pattern is: ^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$ This matches most valid email formats, but complete email validation is complex. For production, use specialized email validation libraries rather than relying solely on regex.

Capture groups are parts of a regex pattern enclosed in parentheses () that "capture" matched text. For example, in "(\d{4})-(\d{2})-(\d{2})" matching "2023-12-25", group 1 captures "2023", group 2 captures "12", and group 3 captures "25". Use (?:...) for non-capturing groups.

Special characters (called metacharacters) like . ^ $ * + ? { } [ ] \ | ( ) have special meanings in regex. To match them literally, escape them with a backslash: \. \^ \$ \* \+ \? \{ \} \[ \] \\ \| \( \). For example, use "\." to match an actual period.

Greedy quantifiers (* + {n,}) match as much as possible, while lazy quantifiers (*? +? {n,}?) match as little as possible. For example, in "abc123def", the pattern "a.*f" (greedy) matches the entire string, while "a.*?f" (lazy) matches just "abc123def".

Common validation patterns: Phone (US): ^\(?([0-9]{3})\)?[-. ]?([0-9]{3})[-. ]?([0-9]{4})$, URL: ^https?:\/\/(www\.)?[-a-zA-Z0-9@:%._\+~#=]{1,256}\.[a-zA-Z0-9()]{1,6}\b([-a-zA-Z0-9()@:%_\+.~#?&//=]*)$, Strong Password: ^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$

Regex syntax varies slightly between languages and engines. JavaScript, Python, Java, and others have different escaping rules, flag syntaxes, and supported features. Test your regex in the specific environment where you'll use it. Our tool uses JavaScript regex engine, which is widely compatible but may differ from other languages.