libphonenumber\PhoneNumberUtil::maybeStripExtension PHP Method

    protected function maybeStripExtension(&$number)
    {
        $matches = array();
        $find = preg_match(static::$EXTN_PATTERN, $number, $matches, PREG_OFFSET_CAPTURE);
        // If we find a potential extension, and the number preceding this is a viable number, we assume
        // it is an extension.
        if ($find > 0 && static::isViablePhoneNumber(substr($number, 0, $matches[0][1]))) {
            // The numbers are captured into groups in the regular expression.
            for ($i = 1, $length = count($matches); $i <= $length; $i++) {
                if ($matches[$i][0] != "") {
                    // We go through the capturing groups until we find one that captured some digits. If none
                    // did, then we will return the empty string.
                    $extension = $matches[$i][0];
                    $number = substr($number, 0, $matches[0][1]);
                    return $extension;
                }
            }
        }
        return "";
    }