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1. Introduction
Some characters are treated specially when processing XML documents. These are the characters which are used to markup XML syntax; when they appear as a part of a document rather than for syntax markup, they need to be appropriately escaped. These characters are:
1 2 3 4 5 | " " Double quote' ' Single quote< < Left angle bracket> > Right angle bracket& & Ampersand |
2. Character Data
All text that is not markup constitutes character data of the document. Within character data, “&” and “<” must not appear except when used in markup. However “>”, “”” and “‘” can appear directly within character data without having to be encoded.
1 2 | <?xml version="1.0"?><valid>"'></valid> |
1 2 | <?xml version="1.0"?><invalid>&</invalid> |
1 2 | <?xml version="1.0"?><invalid><</invalid> |
However, “>” cannot appear in the form “]]>” unless it is a part of CDATA ending sections.
1 2 | <?xml version="1.0"?><invalid>]]></invalid> |
The following is valid as “>” has been encoded properly:
1 2 | <?xml version="1.0"?><valid>]]></valid> |
2. Attributes
Within attributes, “>” is valid.
1 | <valid attrib=">"></valid> |
However, ‘”‘ is not valid within double quotes; it must be encoded using “"”.
1 | <valid attrib="""></valid> |
Similarly “‘” is not valid within single quotes. It must be encoded as “'”:
1 | <valid attrib='''></valid> |
3. Comments
Comments can appear anywhere in a document outside of markup. Within comments, none of the 5 special characters must be escaped or encoded.
1 | <valid><!-- '"<>& --></valid> |
In addition, the string “–” must not appear within comments.
1 | <invalid><!-- Hello -- there --></invalid> |
A consequence of this rule is that a comment must not end with “—>” (three dashes followed by a right-angle-bracket). The following is invalid:
1 | <invalid><!-- Hello there ---></invalid> |
4. Processing Instructions
Processing Instructions (PI) are used to add instructions for XML processing applications. None of the 5 special characters must be encoded within PI statements.
1 | <valid><?execute <>"'& ?></valid> |
A further restriction in the case of processing instructions is that the instruction name must not be the string “xml”; this name is reserved for standardization of the XML specification itself.
1 | <invalid><?xml hello ?></invalid> |
CData
CDATA sections are used to escape blocks of text containing characters which would otherwise be recognized as markup. This section begin with the string “<![CDATA[” and end with the string “]]>”. Within a CData section, none of the 5 special characters must be encoded.
1 | <valid><![CDATA[[<greeting>Hello world: <>&'" </greeting>]]></valid> |
However, within CData sections, the string “]]>” must not appear except to end the section:
1 | <invalid><![CDATA[[This string("]]>") must not appear here]]></invalid> |
The same can be re-written with two CData sections as follows:
1 | <valid><![CDATA[[This string("]]]]><![CDATA[[>") must not appear here]]></valid> |
Conclusion
This article demonstrated what the predefined XML entities are and the various circumstances in which they can be used.