Entity References

Entity references are markup that the parser replaces with character data. In HTML, there are hundreds of predefined character entities, including the Greek alphabet, math symbols, and the copyright symbol. There are only five predefined entity references in XML, however, as shown in Table 3-2.

We've already seen how entity references can be used as content. They can also appear within attribute values. According to Table 3-2,

<CD title="Brooks &amp; Dunn&apos;s Greatest Hits" />

is equivalent to the decimal representation:

<CD title="Brooks &#38; Dunn&#39;s Greatest Hits" />

and to the hexidecimal representation:

<CD title="Brooks &#x26; Dunn&#x27;s Greatest Hits" />

However, the next line is illegal because ampersand ("&") must be escaped by using either the entity reference or one of its numeric representations:

<CD title="Brooks & Dunn&apos;s Greatest Hits" />

This is because ampersand and less-than are special cases.

---

Note: You are required to use the predefined entities < and & to escape the characters < and & in all cases other than when these characters are used as markup delimiters, or in a comment, a processing instruction, or a CDATA section. In other words, the literal "<" and "&" characters can appear only as markup

• delimiters, or within a comment, a processing instruction, or a CDATA section.

Listing 3-3 illustrates the use of all five predefined character entities, several decimal representations of Greek letters, and the three legal variations of the Brooks & Dunn example. If we run this through an XML parser, we can verify that it is well-formed; we did not use the literal ampersand or the literal less-than

<?xml version="1.0" standalone="yes"?>
<Predefined>
  <Test>The hot tip from today&apos;s &lt;StockWatch&gt; column is:
&quot;AT&amp;T stock is doing better than 
Ralph Spoilsports Motors&apos; stock.&quot;
  </Test>
  <PS>Now, wasn&apos;t that as easy as &#928;?
Or &#945;, &#946;, &#947;?</PS>
  <CD title="Brooks &amp; Dunn&apos;s Greatest Hits" />
  <CD title="Brooks &#38; Dunn&#39;s Greatest Hits" />
  <CD title="Brooks &#x26; Dunn&#x27;s Greatest Hits" />
</Predefined>

FIGURE 3-1 Predefined entities displayed in Internet Explorer

before the word StockWatch. Figure 3-1 shows how this example looks in Internet Explorer, which renders the characters that are represented by the entities. It also confirms that the three Brooks & Dunn variations are equivalent.

HTML (and therefore XHTML) includes three large sets of predefined entities: Latin1, Special, and Symbols. You can pull these into your XML document using external entities, covered in chapter 4. The files containing the entities are:

http://www.w3.org/TR/xhtml1/DTD/xhtml-lat1.ent
http://www.w3.org/TR/xhtml1/DTD/xhtml-special.ent
http://www.w3.org/TR/xhtml1/DTD/xhtml-symbol.ent

White Space Is Significant
XML Family of Specifications: A Practical Guide
CDATA Sections