Move point forward, stopping before a char not in STRING, or at pos LIM.
STRING is like the inside of a `[...]' in a regular expression
except that `]' is never special and `\' quotes `^', `-' or `\'
(but not at the end of a range; quoting is never needed there).
Thus, with arg "a-zA-Z", this skips letters stopping before first nonletter.
With arg "^a-zA-Z", skips nonletters stopping before first letter.
Char classes, e.g. `[:alpha:]', are supported.
Returns the distance traveled, either zero or positive.
(fn STRING &optional LIM)