This is not a "good" regex problem: what you really want to do is use a regex to find if a string contains any "special" characters, then count the number of non-special characters. But your description basically comes down to: you can use any characters you like, but you must have at least one non-special character (because you can have specials anywhere, but you need at least one non-special, but you don't have to have any specials.) So use the regex you have - or better, use this:
\w+
which is about the same, but includes more accented characters.