What it does
RegexOne teaches regular expressions through interactive lessons and practice tasks for common matching problems.
Work through character matching and ranges before moving to groups or boundaries. The practice list then gives those patterns tasks such as matching filenames or trimming whitespace. Write a few strings that should fail as well as succeed; this makes an exercise about the pattern's limits rather than only one successful match. The linked language guides cover C#, JavaScript, Java, PHP and Python. Check the guide for the engine you will use, especially when moving a pattern from an exercise into application code.