def format_by_pattern(numobj, number_format, user_defined_formats):
    """Formats a phone number using client-defined formatting rules."

    Note that if the phone number has a country calling code of zero or an
    otherwise invalid country calling code, we cannot work out things like
    whether there should be a national prefix applied, or how to format
    extensions, so we return the national significant number with no
    formatting applied.

    Arguments:
    numobj -- The phone number to be formatted
    num_format -- The format the phone number should be formatted into
    user_defined_formats -- formatting rules specified by clients

    Returns the formatted phone number.
    """
    country_code = numobj.country_code
    nsn = national_significant_number(numobj)
    if not _has_valid_country_calling_code(country_code):
        return nsn
    # Note region_code_for_country_code() is used because formatting
    # information for regions which share a country calling code is contained
    # by only one region for performance reasons. For example, for NANPA
    # regions it will be contained in the metadata for US.
    region_code = region_code_for_country_code(country_code)
    # Metadata cannot be None because the country calling code is valid.
    metadata = PhoneMetadata.metadata_for_region_or_calling_code(country_code, region_code)

    formatted_number = U_EMPTY_STRING
    formatting_pattern = _choose_formatting_pattern_for_number(user_defined_formats, nsn)
    if formatting_pattern is None:
        # If no pattern above is matched, we format the number as a whole.
        formatted_number = nsn
    else:
        num_format_copy = _copy_number_format(formatting_pattern)
        # Before we do a replacement of the national prefix pattern $NP with
        # the national prefix, we need to copy the rule so that subsequent
        # replacements for different numbers have the appropriate national
        # prefix.
        np_formatting_rule = formatting_pattern.national_prefix_formatting_rule
        if np_formatting_rule:
            national_prefix = metadata.national_prefix
            if national_prefix:
                # Replace $NP with national prefix and $FG with the first
                # group (\1) matcher.
                np_formatting_rule = np_formatting_rule.replace(_NP_STRING, national_prefix)
                np_formatting_rule = np_formatting_rule.replace(_FG_STRING, unicod("\\1"))
                num_format_copy.national_prefix_formatting_rule = np_formatting_rule
            else:
                # We don't want to have a rule for how to format the national
                # prefix if there isn't one.
                num_format_copy.national_prefix_formatting_rule = None
        formatted_number = _format_nsn_using_pattern(nsn, num_format_copy, number_format)
    formatted_number = _maybe_append_formatted_extension(numobj,
                                                         metadata,
                                                         number_format,
                                                         formatted_number)
    formatted_number = _prefix_number_with_country_calling_code(country_code,
                                                                number_format,
                                                                formatted_number)
    return formatted_number
Formats a phone number using client-defined formatting rules."

    Note that if the phone number has a country calling code of zero or an
    otherwise invalid country calling code, we cannot work out things like
    whether there should be a national prefix applied, or how to format
    extensions, so we return the national significant number with no
    formatting applied.

    Arguments:
    numobj -- The phone number to be formatted
    num_format -- The format the phone number should be formatted into
    user_defined_formats -- formatting rules specified by clients

    Returns the formatted phone number.