The regex in the last comment has some typos. This is a
syntactically valid one, don't know if it's correct though.
You've to concat the expression in one long line.
^(
[\x00-\x7f]|
[\xc2-\xdf][\x80-\xbf]|
[\xe0][\xa0-\xbf][\x80-\xbf]|
[\xe1-\xec][\x80-\xbf]{2}|
[\xed][\x80-\x9f][\x80-\xbf]|
[\xee-\xef][\x80-\xbf]{2}|
[\xf0][\x90-\xbf][\x80-\xbf]{2}|
[\xf1-\xf3][\x80-\xbf]{3}|
[\xf4][\x80-\x8f][\x80-\xbf]{2}
)*$