error

[nIxedoahz]

[\u4e00-\u9fa5]+ {
yylval->sval = strdup(yytext);
return string;
}

error:
warning that the character range [0-u] is ambiguous in a scanner where case is not required

[Mightyjo]

Flex regex doesn't support Unicode escape sequences. You can construct rules or patterns for code points a byte at a time using hex escape sequences.

The range character class constructor assumes the operands are bytes, so it's not going to work correctly with wide characters. That's what you're seeing.