Editing XML (section)

=== Escaping ===
XML provides ''[[Escape sequence|escape]]'' facilities for including characters that are problematic to include directly. For example:
* The characters "&lt;" and "&" are key syntax markers and may never appear in content outside a [[CDATA]] section. It is allowed, but not recommended, to use "&lt;" in XML entity values.{{sfnp|Bray|Paoli|Sperberg-McQueen|Maler|2008|loc=section 2.3}}
* Some character encodings support only a subset of Unicode. For example, it is legal to encode an XML document in ASCII, but ASCII lacks code points for Unicode characters such as "é".
* It might not be possible to type the character on the author's machine.
* Some characters have [[homoglyph|glyphs]] that cannot be visually distinguished from other characters, such as the [[nonbreaking space]] (<code>&amp;#xa0;</code>) " " and the [[Space (punctuation)|space]] (<code>&amp;#x20;</code>) " ", and the [[А|Cyrillic capital letter A]] (<code>&amp;#x410;</code>) "А" and the [[A|Latin capital letter A]] (<code>&amp;#x41;</code>) "A".

There are five [[List of XML and HTML character entity references#Predefined entities in XML|predefined entities]]:
* <code>&amp;lt;</code> represents "&lt;";
* <code>&amp;gt;</code> represents "&gt;";
* <code>&amp;amp;</code> represents "&";
* <code>&amp;apos;</code> represents "{{mono|'}}";
* <code>&amp;quot;</code> represents '{{mono|"}}'.

All permitted Unicode characters may be represented with a ''[[numeric character reference]]''. Consider the Chinese character "中", whose numeric code in Unicode is hexadecimal 4E2D, or decimal 20,013. A user whose keyboard offers no method for entering this character could still insert it in an XML document encoded either as <code>&amp;#20013;</code> or <code>&amp;#x4e2d;</code>. Similarly, the string "I &lt;3 Jörg" could be encoded for inclusion in an XML document as <code>I &amp;lt;3 J&amp;#xF6;rg</code>.

<code>&amp;#0;</code> is not permitted because the [[null character]] is one of the control characters excluded from XML, even when using a numeric character reference.<ref>{{cite web|first1=Tex|last1=Texin|first2=François|last2=Yergeau|date=6 September 2003|url=http://www.w3.org/International/questions/qa-controls|title=W3C I18N FAQ: HTML, XHTML, XML and Control Codes|website=W3C Internationalization|publisher=W3C|access-date=16 November 2017}}</ref> An alternative encoding mechanism such as [[Base64]] is needed to represent such characters.