Regex Tester — Live Match Highlighting

Email addresses in text need both match and capture

When you have a line like Contact: hello@7busyboss.com or support@example.org, you might only want the domain part, not the full address. Testing \b(\w+)@(\w+)\.(\w+)\b gives you both the whole match and three capture groups — local part, domain name and TLD. This tester shows all of it: each group appears in the match details table so you can see exactly what you would extract in code.

Flags force re-evaluation of your entire pattern

The same pattern behaves differently with each flag. Case-insensitive i makes \bEmail\b match Email, EMAIL or email. The global flag g forces the loop to find every occurrence, not just the first. Multiline m makes ^ and $ match line starts and ends rather than string boundaries. Dotall s lets . match newlines, and u enables Unicode handling. Toggle each and watch the highlights shift. The sticky flag y is not offered.

A pattern that gets stuck is a pattern that hangs your tab

Some patterns cause catastrophic backtracking: the engine tries so many combinations of the same substring that it effectively never returns. (a+)+b tested against a long run of a characters that never reaches a b is the classic case. This tester caps results at 1000 matches, which prevents runaway match loops, but nothing can interrupt the engine inside a single match attempt. If you paste a pathological pattern and the tab locks up, that is what happened — close the tab and simplify the pattern.

Capture groups carry their order, not their names

JavaScript supports named groups via (?<name>pattern), and they work here — but the match table lists groups by position, not by name. Write (?<domain>\w+\.\w+) and the output still labels it group 1. The position is what you index in code, so this is a display limitation rather than a functional one. For validating structured data instead of matching text, see the JSON Formatter.