I once suddenly couldn't log in to a paid bike renting service app because it wouldn't accept my (valid at sign-up) email on login as it had a '+' symbol. There

I once suddenly couldn't log in to a paid bike renting service app because it wouldn't accept my (valid at sign-up) email on login as it had a '+' symbol. There was basically no way to contact the developers and it was impossible to get the support people to understand what my problem was so I literally never used the service again. Don't try to parse email with regex, it's just not worth it.