Check if a string is UTF8 encoded

转载地址： http://blog.qita.in/?post=418

function is_utf8($string){
    return preg_match('%^(?:
         [x09x0Ax0Dx20-x7E]            # ASCII
       | [xC2-xDF][x80-xBF]             # non-overlong 2-byte
       |  xE0[xA0-xBF][x80-xBF]        # excluding overlongs
       | [xE1-xECxEExEF][x80-xBF]{2}  # straight 3-byte
       |  xED[x80-x9F][x80-xBF]        # excluding surrogates
       |  xF0[x90-xBF][x80-xBF]{2}     # planes 1-3
       | [xF1-xF3][x80-xBF]{3}          # planes 4-15
       |  xF4[x80-x8F][x80-xBF]{2}     # plane 16
   )*$%xs', $string);
}