Regex Tester

Regular expressions (regex) are patterns used to match character combinations in strings. They are essential for search, validation, extraction, and text transformation. A regex pattern is written with special syntax: literal characters match themselves, while metacharacters like . * + ? and character classes like [a-z] match categories of characters.

This tester lets you write a pattern, toggle flags (global, case-insensitive, multiline), and paste any test string. Every match is highlighted inline, and each match's capture groups are shown with their exact positions. The tool validates the pattern as you type and shows syntax errors immediately.

Regex flags control how the pattern behaves. The g (global) flag finds all matches instead of just the first. The i (case-insensitive) flag ignores case. The m (multiline) flag makes ^ and $ match line boundaries instead of string boundaries. The s (dotAll) flag makes . match newlines. The u (unicode) flag enables full Unicode support.

Regex evaluation uses the browser's native JavaScript RegExp engine. Matches are found using RegExp.exec() in a loop when the global flag is set, or RegExp.test() for single-match mode.

Capture groups are extracted from the match's groups property. Named groups are labeled; numbered groups are indexed starting from 1. The full match is index 0.

Match positions are calculated using the match's index property (start) and index + match.length (end). This is exact — no estimation.

Pattern validation is performed by attempting to construct a RegExp object. If the constructor throws, the pattern is invalid and the error message is displayed.