Regex Tester

A regular expression (regex or regexp) is a sequence of characters that defines a search pattern in text. They are a formal language derived from automata theory and formal language theory. With regex you can search, validate, extract, and replace text with complex patterns in a single line of code. They are used in every programming language, text editors, databases, and command-line tools.

The most commonly used patterns include: email (^[\w.+-]+@[\w-]+\.[\w.]+$), URL (https?://[\S]+), phone (\+?[\d\s\-()]{7,15}), ZIP code (/^\d{5}(-\d{4})?$/), ISO date (\d{4}-\d{2}-\d{2}), IPv4 (\b(?:\d{1,3}\.){3}\d{1,3}\b) and alphanumeric only ([a-zA-Z0-9]+). These patterns are starting points — in production they should always be adapted to the exact expected format.

Flags modify the behavior of the pattern: g (global) finds all matches instead of stopping at the first; i (insensitive) ignores case; m (multiline) makes ^ and $ match the start and end of each line, not just the whole text; s (dotAll) makes the dot (.) match newlines too, which it excludes by default. Flags can be combined: /pattern/gim.

Capture groups, defined with parentheses (), let you extract specific parts of a match. For example, /(\d{4})-(\d{2})-(\d{2})/ on '2024-03-15' captures three groups: year, month, and day. Named groups (?<name>pattern) allow access by name rather than numeric index. Non-capturing groups (?:pattern) group without capturing, useful for applying quantifiers without creating a group.

Lookahead and lookbehind are position assertions that check what comes before or after a position without consuming characters. Positive lookahead (?=pattern): matches if followed by pattern. Negative lookahead (?!pattern): matches if NOT followed by pattern. Positive lookbehind (?<=pattern): matches if preceded by pattern. Negative lookbehind (?<!pattern): matches if NOT preceded by pattern. For example, \d+(?= dollars) extracts numbers followed by ' dollars' without including the word.