Illegal PCDATA Characters and Escaping Characters : Entity « Introduction « XML Tutorial

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<!--This is not well-formed XML!--> 
<comparison>6 is < 7 & 7 > 6</comparison> 

You can't put < or & characters into PCDATA. 
There are two ways you can get around this: escaping characters, or enclosing text in a CDATA section. 

To escape the < or &, you replace any < with &lt; and any & with &amp;. 
The previous XML example could be made well formed: 

<comparison>6 is &lt; 7 &amp; 7 &gt; 6 </comparison> 

&lt; and &amp; are known as entity references. 
The following entities are defined in XML: 

&amp;    - the & character 
&lt;     - the < character 
&gt;     - the > character 
&apos;   - the ` character 
&quot;   - the " character 

Other characters can also be escaped by using character references. 
These are strings such as &#nnn;, where nnn would be replaced by the Unicode number of the character. 
Or &#xnnn; with an x preceding the number, where nnn is a hexadecimal representation of the Unicode character.
To include the copyright symbol ((c)),  inserting &#169; or &#xA9;. 
Entity references must be declared