If you encode them in UTF-8, they are allowed. In UTF-8, to represent a given code point, only the first code unit can be in the range 0x00 to 0x7F. So if you're reading UTF-8 content and find a byte with value 0x00-0x7F, you can be sure it is a 1-byte long code unit sequence representing the code points U+0000 to U+007F.