Type a regular expression, paste some test text, and see every match highlighted as you go — with positions and capture groups listed underneath.
What is a regular expression? A pattern language for finding text: \d means any digit, \w any word character, + means "one or more of the previous thing", and ^ and $ anchor to the start and end. Developers use them to validate formats, search logs, and pull pieces out of text.
Why does it find only one match? Without the g flag, a search stops at the first match. This tester looks for all matches automatically when building the match list — the g checkbox controls the highlighting behaviour as most regex tools present it.