Based on the XML spec at http://www.w3.org/TR/xml/#charsets, the valid characters are:

```
Char	   ::=   	#x9 | #xA | #xD | [#x20-#xD7FF] | [#xE000-#xFFFD] | [#x10000-#x10FFFF]
```

This explicitly excludes the surrogate character ranges.