def _is_allowed_abbr(self, tokens):
        """Return True if text is an allowed abbreviation."""
        if len(tokens) <= 2:
            abbr_text = ''.join(tokens)
            if self.abbr_min <= len(abbr_text) <= self.abbr_max and bracket_level(abbr_text) == 0:
                if abbr_text[0].isalnum() and any(c.isalpha() for c in abbr_text):
                    # Disallow property values
                    if re.match('^\d+(\.\d+)?(g|m[lL]|cm)$', abbr_text):
                        return False
                    return True
        return False
Return True if text is an allowed abbreviation.