Example usage of validate.pattern of a form field

Has anyone a, or preferably more than one, example of a validation pattern to be used to validate an entered value in a Form Plugin field?
Searching the web and of course including this forum and the getgrav repo's on GitHub didn't help. Info on this seems very rare?

Any valid regex pattern should work, like here. Can't find any good examples of it in use either, but the web has plenty of examples of regex-rules, and you could use a tool like Regex101 to test values against patterns.

Great, thanks. I overlooked that one, probably because I'm more busy with front-end forms.