Issue using regular expression validator

I have a text box with both a required field and regular expression validator which should only contain alphanumeric, spaces, dashes, and underscores. This is what i put in the validation expression : "[a-zA-Z0-9\s-_]" . I tested it it out by entering letters, numbers, spaces, and dashes into the box but it had failed the validation. The odd thing is that if i put only a single character like a letter or number into the box, it passed. What could be causing this behavior?