Why do I get a syntax error with REXML using this code?

I am using Ruby's REXML library with this code:

  VALID_XML_CHARS = /^(
       [\x09\x0A\x0D\开发者_Python百科x20-\x7E]            # ASCII
     | [\xC2-\xDF][\x80-\xBF]             # non-overlong 2-byte
     |  \xE0[\xA0-\xBF][\x80-\xBF]        # excluding overlongs
     | [\xE1-\xEC\xEE][\x80-\xBF]{2}      # straight 3-byte
     |  \xEF[\x80-\xBE]{2}                #
     |  \xEF\xBF[\x80-\xBD]               # excluding U+fffe and U+ffff
     |  \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
     )*$/x;

I get a syntax error in the last line, and I am not sure why. Any ideas?

Well the place where you seem to have gotten this from (here) ends it with /nx;