'Parse a "-delimited string or \'-delimited link.
The offset must be pointing to either a single quote ("\'") or double
quote (\'"\') character. The strings are parsed according to Excel
rules where to escape the delimiter you just double it up. E.g.,
"abc""def" in Excel is parsed as \'abc"def\' in Python.
Returns the number of characters matched. (Does not update
self.offset)'

def parse_string(self):

    self.assert_empty_token()
    delim = self.formula[self.offset]
    assert (delim in ('"', "'"))
    regex = self.STRING_REGEXES[delim]
    match = regex.match(self.formula[self.offset:])
    if (match is None):
        subtype = ('string' if (delim == '"') else 'link')
        raise TokenizerError(('Reached    end    of    formula    while    parsing    %s    in    %s' % (subtype, self.formula)))
    match = match.group(0)
    if (delim == '"'):
        self.items.append(Token.make_operand(match))
    else:
        self.token.append(match)
    return len(match)
