Checking String for illegal characters using regular expression

Question

I want to check a for any illegal character using the following regular expression in PHP. Essentially, I want to allow only alphanumeric and underscore (_). Unfortunately the

Answer 1

You have no repeater for one. You need a repeater such as +. As far as I can see without executing it, you check start of line and one character matching a-zA-Z0-9 and _ but nothing following that first character.