Escape hyphen in character set

The old one didn't work with http://regexpal.com/ so I fixed it according to https://stackoverflow.com/questions/3697202/including-a-hyphen-in-a-regex-character-bracket. Probably doesn't matter if it already works.

Disclaimer: I'm a noob at git/github, so please let me know if I made the pull request incorrectly or something :P