Learning XML, 2nd Edition

Miscellaneous Markup

Rounding out the list of markup objects are comments, processing instructions, and CDATA sections. They all have one thing in common: they shield content from the parser in some fashion. Comments keep text from ever getting to the parser. CDATA sections turn off the tag resolution, and processing instructions target specific processors.

Comments

Comments are notes in the document that are not interpreted by the XML processor. If you’re working with other people on the same files, these messages can be invaluable. They can be used to identify the purpose of files and sections to help navigate a cluttered document, or simply to communicate with each other.

Figure 2-21 shows the form of a comment. It starts with the delimiter <!-- (1) and ends with the delimiter --> (3). Between these delimiters goes the comment text (2) which can be just about any kind of text you want, including spaces, newlines, and markup. The only string not allowed inside a comment is two or more dashes in succession, since the parser would interpret that string as the end of the comment.

Figure 2-21. Comment syntax

Comments can go anywhere in your document except before the XML declaration and inside tags. The XML processor removes them completely before parsing begins. So this piece of XML:

<p>The quick brown fox jumped<!-- test -->over the lazy dog. The quick brown <!-- test --> fox jumped over ...