And interesting one to stumble upon – the ‘official‘ regex for validating a email address:
Edit: I’ve had to break this down into parts – for some reason WordPress throws a 403 error if I don’t. I think it sees this regex as a security risk!
Before the @ sign:
The @ sign and after:
Reads like it should work pretty well to me. I didn’t realise before the @ was so generous. And you could shrink it by using a case insensitive operator.