How to detect UTF-8 in plain C?

I am looking for a code snippet in plain old C that detects that the given string is in UTF-8 encoding. I know the solution with regex, but for various reasons it would be better to avoid using anything but plain C in this particular case.

Solution with regex looks like this (warning: various checks omitted):

#define UTF8_DETECT_REGEXP  "^([\x09\x0A\x0D\x20-\x7E]|[\xC2-\xDF][\x80-\xBF]|\xE0[\xA0-\xBF][\x80-\xBF]|[\xE1-\xEC\xEE\xEF][\x80-\xBF]{2}|\xED[\x80-\x9F][\x80-\xBF]|\xF0[\x90-\xBF][\x80-\xBF]{2}|[\xF1-\xF3][\x80-\xBF]{3}|\xF4[\x80-\x8F][\x80-\xBF]{2})*$"const char *error;int         error_off;int         rc;int         vect[100];utf8_re = pcre_compile(UTF8_DETECT_REGEXP, PCRE_CASELESS, &error, &error_off, NULL);utf8_pe = pcre_study(utf8_re, 0, &error);rc = pcre_exec(utf8_re, utf8_pe, str, len, 0, 0, vect, sizeof(vect)/sizeof(vect[0]));if (rc > 0) {    printf("string is in UTF8\n");} else {    printf("string is not in UTF8\n")}