Cosmetic regex change.

Added a backslash before the [ in the character class to make Emacs' syntax highlighter happy. Yes, it's Emacs' fault, and I know that it's not strictly necessary. But that missing backslash there totally throws off Emacs' syntax highlighting so that the whole rest of the file looks like one big string which doesn't help editing that file in Emacs. :) Since I do all my editing in Emacs I'd much appreciate it if this could be merged upstream, thanks a lot in advance!