def exists(self, regex):
return self.search_full(regex, return_string=False, advance_pointer=False)<EOL>
See what :meth:`skip_until` would return without advancing the pointer.

    >>> s = Scanner("test string")
    >>> s.exists(' ')
    5
    >>> s.pos
    0

Returns the number of characters matched if it does exist, or ``None``
otherwise.