Regex Letters, Numbers, Dashes, and Underscores
Just escape the dashes to prevent them from being interpreted (I don't think underscore needs escaping, but it can't hurt). You don't say which regex you are using.
([A-Za-z0-9\-\_]+)
Your expression should already match dashes, because the final - will not be interpreted as a range operator (since the range has no end). To add underscores as well, try:
([A-Za-z0-9_-]+)