What's the best way to detect forbidden chars in string?

Right now, I am using the find function and search through a string for any character that is forbidden. I only allow 0-9, A-Z, a-z, and — While using the find function for all the forbidden characters do work, the checklist itself is long and it looks ugly. Is there a better way to achieve this? Basically a function that returns -1 if there is any other character beside 0-9, a-Z, A-Z and —

this would work:

If you want to know if what is entered is a valid e-mail, you should consider using Regex

https://www.scirra.com/tutorials/1337/e ... t-2/page-1

And here's the expression used in Regex to find a valid e-mail-adress:

http://www.regular-expressions.info/email.html

\b[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,4}\b