Regular expression to find valid UTF-8 characters (useful for filtering bad input which might break some applications)

valid-utf-8-regexp.txt

[\x00-\x7F]                         # 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