Invalid characters in XML

Your question addresses multiple topics.
I ll try to give a brief explanation of these topics

in an XML stream are the characters allowed that are mentioned in the above definition
#x9 is the ninth character as defined in Unicode
The codechars you can find here
http://www.unicode.org/Public/4.1.0/charts/CodeCharts.pdf (30 MegaByte)

Not allowed in an XML stream are the first 32 characters in this set
(including NUL(0), excluding "tab"(9), "linefeed" (10) and "carriage return"(13))
This means 29 characters are not allowed
Then there are a number of characters not allowed in the higher blocks starting at character 55296
You can count them your self from the above definition of Char.

The codecharts are just a mapping between a character and a numerical representation.
How they are stored on the computer depends on the encoding.
At this point, number of bits/bytes and byte order come into play

When you ask "what is the corresponding Hex value"? I assume you want the encoding question answered. Well this depends on the encoding.

At the start of your XML document you can have this string
<?xml version="1.0" encoding="ISO-8859-1" standalone="yes"?>
The ISO-8859-1 encoding is the ISO standardised Latin one (Ascii). That characterset encodes 1byte, so 255 characters and these 255 characters byte numbers match the first part of the unicode standard, so that is easy

This means that with the iso latin encoding you cannot express a "h with a ^" (character 293 or x125) except with a character entity, like this "&#x125;" or "&#293;" (using 6/7 bytes)

If this xml declaration is not present, UTF-8 is assumed as encoding.
UTF8 uses one byte for the first 127 characters (exact match to ascii as well) but uses two bytes for the next part (one special code byte plus another for counting:
in iso latin 1 "é" would be one byte, 233
in UTF-8, "é" would be two bytes

If you are pulling information streams from a mainframe. You have to know the meaning of each byte value on the mainframe, map it to the encoding you pick for the XML
and remove or escape the characters that are not allowed

On a higher level an XML stream consists of markup and character data.
Since markup uses some special characters "<", ">" and "&". These need to be escaped.
A "<" or a "&" in a character data part makes the XML unvalid as well...

I hope this is a start.
Happy to provide you with more info if required

Geert
here you need to escape "<" by "&lt;" and "&" by "&amp;"