C0 Control Characters
The first 32 Unicode characters with code points from 0 to 31 are known as the C0 controls. They were originally defined in ASCII to control teletypes and other monospace dumb terminals. Aside from the tab, carriage return, and line feed they have no obvious meaning in text. Since XML is text, it does not include binary characters such as NULL (#x00), BEL (#x07), DC1 (#x11) through DC4 (#x14), and so forth. These noncharacters are historical relics. XML 1.0 does not allow them. This is a good thing. Although dumb terminals and binary-hostile gateways are far less common today than they were twenty years ago, they are still used, and passing these characters through equipment that expects to see plain text can have nasty ...
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