Regex Tester

A regular expression (regex) is a sequence of characters that defines a search pattern. Regex is used for string matching, validation, search and replace, and text parsing. It is supported in virtually every programming language and many text editors.

Common regex patterns include: email validation (^[\w.-]+@[\w.-]+\.\w{2,}$), phone numbers (\d{3}-\d{3}-\d{4}), URLs (https?://[\w.-]+), IP addresses (\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}), and dates (\d{4}-\d{2}-\d{2}).

Regex flags modify how the pattern is applied: g (global) finds all matches instead of stopping at the first, i (case-insensitive) ignores letter case, m (multiline) makes ^ and $ match line boundaries instead of string boundaries, and s (dotall) makes the dot (.) match newline characters.

Capture groups are created by wrapping part of a regex pattern in parentheses (). They allow you to extract specific parts of a match. For example, (\d{4})-(\d{2})-(\d{2}) applied to "2024-01-15" captures three groups: "2024", "01", and "15". Non-capturing groups (?:...) match without capturing.

Lookahead (?=...) and lookbehind (?<=...) are zero-width assertions that match a position without consuming characters. Positive lookahead (?=abc) matches if followed by abc. Negative lookahead (?!abc) matches if NOT followed by abc. Lookbehind works similarly but checks what comes before the current position.