What characters do I need to escape in XML documents?

March 13, 2021 by Code Error

What characters must be escaped in XML documents, or where could I find such a list?

Solution

If you use an appropriate class or library, they will do the escaping for you. Many XML issues are caused by string concatenation.

There are only five:

"   &quot;
'   &apos;
<   &lt;
>   &gt;
&   &amp;

Escaping characters depends on where the special character is used.

The examples can be validated at the W3C Markup Validation Service.

The safe way is to escape all five characters in text. However, the three characters ", ' and > needn’t be escaped in text:

<?xml version="1.0"?>
<valid>"'></valid>

The safe way is to escape all five characters in attributes. However, the > character needn’t be escaped in attributes:

<?xml version="1.0"?>
<valid attribute=">"/>

The ' character needn’t be escaped in attributes if the quotes are ":

<?xml version="1.0"?>
<valid attribute="'"/>

Likewise, the " needn’t be escaped in attributes if the quotes are ':

<?xml version="1.0"?>
<valid attribute='"'/>

All five special characters must not be escaped in comments:

<?xml version="1.0"?>
<valid>
<!-- "'<>& -->
</valid>

All five special characters must not be escaped in CDATA sections:

<?xml version="1.0"?>
<valid>
<![CDATA["'<>&]]>
</valid>

All five special characters must not be escaped in XML processing instructions:

<?xml version="1.0"?>
<?process <"'&> ?>
<valid/>

HTML has its own set of escape codes which cover a lot more characters.

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