def _create_extn_pattern(single_extn_symbols):
    # There are three regular expressions here. The first covers RFC 3966
    # format, where the extension is added using ";ext=". The second more
    # generic one starts with optional white space and ends with an optional
    # full stop (.), followed by zero or more spaces/tabs and then the numbers
    # themselves. The other one covers the special case of American numbers
    # where the extension is written with a hash at the end, such as "- 503#".
    # Note that the only capturing groups should be around the digits that you
    # want to capture as part of the extension, or else parsing will fail!
    # Canonical-equivalence doesn't seem to be an option with Android java, so
    # we allow two options for representing the accented o - the character
    # itself, and one in the unicode decomposed form with the combining acute
    # accent.
    return (_RFC3966_EXTN_PREFIX + _CAPTURING_EXTN_DIGITS + u("|") +
            u("[ \u00A0\\t,]*(?:e?xt(?:ensi(?:o\u0301?|\u00F3))?n?|") +
            u("\uFF45?\uFF58\uFF54\uFF4E?|") +
            u("[") + single_extn_symbols + u("]|int|anexo|\uFF49\uFF4E\uFF54)") +
            u("[:\\.\uFF0E]?[ \u00A0\\t,-]*") + _CAPTURING_EXTN_DIGITS + u("#?|") +
            u("[- ]+(") + _DIGITS + u("{1,5})#"))
Helper initialiser method to create the regular-expression pattern to
dataset/ETHPy150Open daviddrysdale/python-phonenumbers/python/phonenumbers/phonenumberutil.py/_create_extn_pattern