Numeric character reference: Difference between revisions

Revision as of 04:53, 17 September 2005

Revision as of 08:02, 2 December 2005

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== Example ==

In SGML, HTML, and XML, the following are all valid numeric character references for the Greek capital letter Sigma ("ΣΣ"):

Σ

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While the syntax of SGML does not prohibit references to unassigned code points, such as <code>&amp;#xFFFF;</code>, SGML-derived markup languages such as HTML and XML can, and often do, restrict numeric character references to only those code points that are assigned to characters or that have not been permanently left unassigned.

Restrictions may also apply for other reasons. For example, in HTML 4, <code>&amp;#12;</code>, which is a reference to a non-printing "form feed" control character, is allowed because a form feed character is allowed. But in XML, the form feed character cannot be used, not even by reference. As another example, <code>&amp;#128;</code>, which is a reference to another control character, is not allowed to be used or referenced in either HTML or XML, but when used in HTML, it is usually not flagged as an error by web browsers&mdash;some of which attempt to interpret it as a reference to the character represented by code value 128 in the [[Windows-1252]] encoding: "&#8364;€", which actually should be represented as <code>&amp;#8364;</code>. As a further example, prior to the publication of XML 1.0 Second Edition on [[October 6]], [[2000]], XML 1.0 was based on an older version of ISO 10646 and prohibited using characters above U+FFFD, except in character data, thus making a reference like <code>&amp;#65536;</code> (U+10000) illegal. In XML 1.1 and newer editions of XML 1.0, such a reference is allowed, because the available character repertoire was explicitly extended.

Markup languages also place restrictions on where character references can occur.