A regular expression, or regex, is a pattern that describes text: an email address, a phone number, a date, or any other shape of characters. They look intimidating at first, because they are written in symbols, but each symbol has a simple job. Once you understand two real patterns, the rest becomes much easier to read.
An email pattern, piece by piece
A simple email pattern checks for characters, then an at sign, then more characters, then a dot, then a domain ending. The key idea is that the pattern does not list every possible email, it describes the shape an email must have. Testing it live, with matches highlighted, shows immediately whether it accepts the addresses it should and rejects the ones it should not.
A phone pattern, piece by piece
A phone pattern works the same way: it allows digits, and optionally spaces, dashes or a country code, in a certain order and count. The difference between a good phone pattern and a bad one is usually how strict it is: too strict and it rejects real numbers, too loose and it accepts anything.
Try it yourself
- Open the Regex Tester and paste a few real examples, plus a few wrong ones, as test text.
- Type the pattern and watch which examples match and which do not.
- Change one part of the pattern at a time, and see how the matches change. That is how you learn what each part does.
Test with wrong examples, not just right ones. A pattern that matches everything is not a validation pattern, it is just a filter that lets everything through.
Everything runs in your browser, so your test text is never uploaded. Start with email and phone, and you will have the foundation for dates, URLs and any other pattern you need next.