The following Perl regular expression tests if a string is well-formed Unicode UTF-8 (Broken up after each | since long lines are not permitted here. Please join as a single line, no spaces, before use.):
^([\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}|
f0[\x90-\xbf][\x80-\xbf]{2}|
[\xf1-\xf3][\x80-\xbf]{3}|
\xf4[\x80-\x8f][\x80-\xbf]{2})*$
NOTE: This strictly follows the Unicode standard 4.0, as described in chapter 3.9, table 3-6, "Well-formed UTF-8 byte sequences" ( http://www.unicode.org/versions/Unicode4.0.0/ch03.pdf#G31703 ).
ISO-10646, a super-set of Unicode, uses UTF-8 (there called "UCS", see http://www.unicode.org/faq/utf_bom.html#1 ) in a relaxed variant that supports a 31-bit space encoded into up to six bytes instead of Unicode's 21 bits in up to four bytes. To check for ISO-10646 UTF-8, use the following Perl regular expression (again, broken up, see above):
^([\x00-\x7f]|
[\xc0-\xdf][\x80-\xbf]|
[\xe0-\xef][\x80-\xbf]{2}|
[\xf0-\xf7][\x80-\xbf]{3}|
[\xf8-\xfb][\x80-\xbf]{4}|
[\xfc-\xfd][\x80-\xbf]{5})*$
The following function may be used with above expressions for a quick UTF-8 test, e.g. to distinguish ISO-8859-1-data from UTF-8-data if submitted from a <form accept-charset="utf-8,iso-8859-1" method=..>.
function is_utf8($string) {
return (preg_match('/[insert regular expression here]/', $string) === 1);
}