检查字符串是否是UTF8编码

用于判断某个字符串是否采用UTF8编码

function is_utf8($string){

    return preg_match('%^(?:

         [\x09\x0A\x0D\x20-\x7E]            # ASCII

       | [\xC2-\xDF][\x80-\xBF]             # non-overlong 2-byte

       |  \xE0[\xA0-\xBF][\x80-\xBF]        # excluding overlongs

       | [\xE1-\xEC\xEE\xEF][\x80-\xBF]{2}  # straight 3-byte

       |  \xED[\x80-\x9F][\x80-\xBF]        # excluding surrogates

       |  \xF0[\x90-\xBF][\x80-\xBF]{2}     # planes 1-3

       | [\xF1-\xF3][\x80-\xBF]{3}          # planes 4-15

       |  \xF4[\x80-\x8F][\x80-\xBF]{2}     # plane 16

   )*$%xs', $string);

}