Sorry, I had a typo in my last comment. Corrected regexp:
^([\\x00-\\x7f]|
[\\xc2-\\xdf][\\x80-\\xbf]|
\\xe0[\\xa0-\\xbf][\\x80-\\xbf]|
[\\xe1-\\xec][\\x80-\\xbf]{2}|
\\xed[\\x80-\\x9f][\\x80-\\xbf]|
\\xef[\\x80-\\xbf][\\x80-\\xbd]|
\\xee[\\x80-\\xbf]{2}|
\xf0[\\x90-\\xbf][\\x80-\\xbf]{2}|
[\\xf1-\\xf3][\\x80-\\xbf]{3}|
\\xf4[\\x80-\\x8f][\\x80-\\xbf]{2})*$