Xml – Difference between PCDATA and CDATA in DTD

dtdxml

What is the difference between #PCDATA and #CDATA in DTD?

Best Solution

  • PCDATA is text that will be parsed by a parser. Tags inside the text will be treated as markup and entities will be expanded.
  • CDATA is text that will not be parsed by a parser. Tags inside the text will not be treated as markup and entities will not be expanded.

By default, everything is PCDATA. In the following example, ignoring the root, <bar> will be parsed, and it'll have no content, but one child.

<?xml version="1.0"?>
<foo>
<bar><test>content!</test></bar>
</foo>

When we want to specify that an element will only contain text, and no child elements, we use the keyword PCDATA, because this keyword specifies that the element must contain parsable character data – that is , any text except the characters less-than (<) , greater-than (>) , ampersand (&), quote(') and double quote (").

In the next example, <bar> contains CDATA. Its content will not be parsed and is thus <test>content!</test>.

<?xml version="1.0"?>
<foo>
<bar><![CDATA[<test>content!</test>]]></bar>
</foo>

There are several content models in SGML. The #PCDATA content model says that an element may contain plain text. The "parsed" part of it means that markup (including PIs, comments and SGML directives) in it is parsed instead of displayed as raw text. It also means that entity references are replaced.

Another type of content model allowing plain text contents is CDATA. In XML, the element content model may not implicitly be set to CDATA, but in SGML, it means that markup and entity references are ignored in the contents of the element. In attributes of CDATA type however, entity references are replaced.

In XML, #PCDATA is the only plain text content model. You use it if you at all want to allow text contents in the element. The CDATA content model may be used explicitly through the CDATA block markup in #PCDATA, but element contents may not be defined as CDATA per default.

In a DTD, the type of an attribute that contains text must be CDATA. The CDATA keyword in an attribute declaration has a different meaning than the CDATA section in an XML document. In a CDATA section all characters are legal (including <,>,&,' and " characters), except the ]]> end tag.

#PCDATA is not appropriate for the type of an attribute. It is used for the type of "leaf" text.

#PCDATA is prepended by a hash in the content model to distinguish this keyword from an element named PCDATA (which would be perfectly legal).