XML Comments and Processing Instructions

Estimated study time: 9 minutes.

Beyond data, XML documents can carry two kinds of non-data content: comments for human readers, and processing instructions for applications that consume the file.

XML Comments

Comments let you annotate a document without affecting how it's parsed as data.

<!-- This section lists enrolled students -->
<students>
  <student>Meera Iyer</student>
</students>

Comment Rules

• Comments cannot appear before the XML declaration.
• Comments cannot be nested inside other comments.
• The character sequence -- cannot appear inside a comment's text.

Processing Instructions (PIs)

A processing instruction passes application-specific information to whatever software is reading the XML — commonly used to link a stylesheet.

<?xml-stylesheet type="text/xsl" href="course-style.xsl"?>
<course>
  <title>Data Analytics</title>
</course>

PIs vs Comments

• Comments are ignored by parsers — they're purely for humans reading the source.
• Processing instructions are passed through to the application, which decides how to act on them.