Sets the list of characters (character class) that are considered part of a word, when selecting by word. The class is defined the same was as a regular expression character class (as normally defined using []'s, but without those included). A leading or trailing hypen (-) is used to include a hyphen in the character class.
Passing a NULL class restores the default behaviour of alphanumerics plus "_" (i.e. "A-Za-z0-9_").