lua 字符串数学表达式运算

function charMathOperation(str)
function chartoint(chstr)
chstr=string.sub(chstr,2,2);
return tostring(string.byte(chstr));
end
str=string.gsub(str,"%b''",chartoint);
str="return "..str;
return loadstring(str)();
end

print(charMathOperation("'a'-'a'"));