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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _extract_possible_number(number):
 """Attempt to extract a possible number from the string passed in. This currently strips all leading characters that cannot be used to start a phone number. Characters that can be used to start a phone number are defined in the VALID_START_CHAR_PATTERN. If none of these characters are found in the number passed in, an empty string is returned. This function also attempts to strip off any alternative extensions or endings if two or more are present, such as in the case of: (530) 583-6985 x302/x2303. The second extension here makes this actually two phone numbers, (530) 583-6985 x302 and (530) 583-6985 x2303. We remove the second extension so that the first number is parsed correctly. Arguments: number -- The string that might contain a phone number. Returns the number, stripped of any non-phone-number prefix (such as "Tel:") or an empty string if no character used to start phone numbers (such as + or any digit) is found in the number """

    match = _VALID_START_CHAR_PATTERN.search(number)
    if match:
        number = number[match.start():]
        # Remove trailing non-alpha non-numberical characters.
        trailing_chars_match = _UNWANTED_END_CHAR_PATTERN.search(number)
        if trailing_chars_match:
            number = number[:trailing_chars_match.start()]
        # Check for extra numbers at the end.
        second_number_match = _SECOND_NUMBER_START_PATTERN.search(number)
        if second_number_match:
            number = number[:second_number_match.start()]
        return number
    else:
        return U_EMPTY_STRING