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{{Short description|Common markup construct used in SGML, XML, and HTML}}
{{one source|date=February 2021}}
A '''numeric character reference''' ('''NCR''') is a common [[markup (computer programming)|markup]] construct used in [[SGML]] and SGML-derived markup languages such as [[HTML]] and [[XML]]. It consists of a short sequence of [[character (computing)|character]]s that, in turn, represents a single character. Since [[SGML|WebSgml]], [[XML]] and [[HTML 4]], the code points of the [[Universal Character Set]] (UCS) of [[Unicode]] are used. NCRs are typically used in order to represent characters that are not [[plain text#Encoding|directly encodable]] in a particular document (for example, because they are international characters that do not fit in the 8-bit [[Character encoding|character set]] being used, or because they have special syntactic meaning in the language). When the document is interpreted by a markup-aware reader, each NCR is treated as if it were the character it represents.
==
In SGML, HTML, and XML, the following are all valid numeric character references for the Greek capital letter Sigma
{| class="wikitable
|+ Numerical character reference of
|-
! [[Unicode#Upluslink|Unicode character]]
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|-
| U+03A3 || Hexadecimal || Σ || Σ
|-
| U+03A3 || Hexadecimal || Σ || Σ
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In SGML, HTML, and XML, the following are all valid numeric character references for the Latin capital letter AE
{| class="wikitable
|+ Numerical character reference of {{unichar|00C6|Latin capital letter AE}}
|-
! [[Unicode#Upluslink|Unicode character]]
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|-
| U+00C6 || Decimal || Æ || Æ
|-
| U+00C6 || Hexadecimal || Æ || Æ
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In SGML, HTML, and XML, the following are all valid numeric character references for the Latin small letter sharp s ß
{| class="wikitable
|+ Numerical character reference of {{unichar|00DF|Latin small letter sharp s}}
|-
! [[Unicode#Upluslink|Unicode character]]
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|-
| U+00DF || Decimal || ß || ß
|-
| U+00DF || Hexadecimal || ß || ß
|}
List of numeric character references for the printable [[ASCII]] characters:
{| class="wikitable"
! [[Unicode#Upluslink|Unicode character]]
! Character<br />Reference<br />(decimal)
! Character<br />Reference<br />(hexadecimal)
! Effect
|-
| U+0020 || &#32; || &#x20; || (space)
|-
| U+0021 || &#33; || &#x21; || !
|-
| U+0022 || &#34; || &#x22; || "
|-
| U+0023 || &#35; || &#x23; || #
|-
| U+0024 || &#36; || &#x24; || $
|-
| U+0025 || &#37; || &#x25; || %
|-
| U+0026 || &#38; || &#x26; || &
|-
| U+0027 || &#39; || &#x27; || '
|-
| U+0028 || &#40; || &#x28; || (
|-
| U+0029 || &#41; || &#x29; || )
|-
| U+002A || &#42; || &#x2A; || *
|-
| U+002B || &#43; || &#x2B; || +
|-
| U+002C || &#44; || &#x2C; || ,
|-
| U+002D || &#45; || &#x2D; || -
|-
| U+002E || &#46; || &#x2E; || .
|-
| U+002F || &#47; || &#x2F; || /
|-
| U+0030 || &#48; || &#x30; || 0
|-
| U+0031 || &#49; || &#x31; || 1
|-
| U+0032 || &#50; || &#x32; || 2
|-
| U+0033 || &#51; || &#x33; || 3
|-
| U+0034 || &#52; || &#x34; || 4
|-
| U+0035 || &#53; || &#x35; || 5
|-
| U+0036 || &#54; || &#x36; || 6
|-
| U+0037 || &#55; || &#x37; || 7
|-
| U+0038 || &#56; || &#x38; || 8
|-
| U+0039 || &#57; || &#x39; || 9
|-
| U+003A || &#58; || &#x3A; || :
|-
| U+003B || &#59; || &#x3B; || ;
|-
| U+003C || &#60; || &#x3C; || <
|-
| U+003D || &#61; || &#x3D; || =
|-
| U+003E || &#62; || &#x3E; || >
|-
| U+003F || &#63; || &#x3F; || ?
|-
| U+0040 || &#64; || &#x40; || @
|-
| U+0041 || &#65; || &#x41; || A
|-
| U+0042 || &#66; || &#x42; || B
|-
| U+0043 || &#67; || &#x43; || C
|-
| U+0044 || &#68; || &#x44; || D
|-
| U+0045 || &#69; || &#x45; || E
|-
| U+0046 || &#70; || &#x46; || F
|-
| U+0047 || &#71; || &#x47; || G
|-
| U+0048 || &#72; || &#x48; || H
|-
| U+0049 || &#73; || &#x49; || I
|-
| U+004A || &#74; || &#x4A; || J
|-
| U+004B || &#75; || &#x4B; || K
|-
| U+004C || &#76; || &#x4C; || L
|-
| U+004D || &#77; || &#x4D; || M
|-
| U+004E || &#78; || &#x4E; || N
|-
| U+004F || &#79; || &#x4F; || O
|-
| U+0050 || &#80; || &#x50; || P
|-
| U+0051 || &#81; || &#x51; || Q
|-
| U+0052 || &#82; || &#x52; || R
|-
| U+0053 || &#83; || &#x53; || S
|-
| U+0054 || &#84; || &#x54; || T
|-
| U+0055 || &#85; || &#x55; || U
|-
| U+0056 || &#86; || &#x56; || V
|-
| U+0057 || &#87; || &#x57; || W
|-
| U+0058 || &#88; || &#x58; || X
|-
| U+0059 || &#89; || &#x59; || Y
|-
| U+005A || &#90; || &#x5A; || Z
|-
| U+005B || &#91; || &#x5B; || [
|-
| U+005C || &#92; || &#x5C; || \
|-
| U+005D || &#93; || &#x5D; || ]
|-
| U+005E || &#94; || &#x5E; || ^
|-
| U+005F || &#95; || &#x5F; || _
|-
| U+0060 || &#96; || &#x60; || '
|-
| U+0061 || &#97; || &#x61; || a
|-
| U+0062 || &#98; || &#x62; || b
|-
| U+0063 || &#99; || &#x63; || c
|-
| U+0064 || &#100; || &#x64; || d
|-
| U+0065 || &#101; || &#x65; || e
|-
| U+0066 || &#102; || &#x66; || f
|-
| U+0067 || &#103; || &#x67; || g
|-
| U+0068 || &#104; || &#x68; || h
|-
| U+0069 || &#105; || &#x69; || i
|-
| U+006A || &#106; || &#x6A; || j
|-
| U+006B || &#107; || &#x6B; || k
|-
| U+006C || &#108; || &#x6C; || l
|-
| U+006D || &#109; || &#x6D; || m
|-
| U+006E || &#110; || &#x6E; || n
|-
| U+006F || &#111; || &#x6F; || o
|-
| U+0070 || &#112; || &#x70; || p
|-
| U+0071 || &#113; || &#x71; || q
|-
| U+0072 || &#114; || &#x72; || r
|-
| U+0073 || &#115; || &#x73; || s
|-
| U+0074 || &#116; || &#x74; || t
|-
| U+0075 || &#117; || &#x75; || u
|-
| U+0076 || &#118; || &#x76; || v
|-
| U+0077 || &#119; || &#x77; || w
|-
| U+0078 || &#120; || &#x78; || x
|-
| U+0079 || &#121; || &#x79; || y
|-
| U+007A || &#122; || &#x7A; || z
|-
| U+007B || &#123; || &#x7B; || {
|-
| U+007C || &#124; || &#x7C; || {{pipe}}
|-
| U+007D || &#125; || &#x7D; || }
|-
| U+007E || &#126; || &#x7E; || ~
|}
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While the syntax of SGML does not prohibit references to invalid or unassigned code points, such as <code>&#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.
Restrictions may also apply for other reasons. For example, in HTML 4, <code>&#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.<ref>{{cite web |title=HTML 5.2: 8. The HTML syntax |url=https://www.w3.org/TR/2017/WD-html52-20170228/syntax.html |website=www.w3.org}}</ref>{{Citation needed|date=May 2013}}
Markup languages also place restrictions on where character references can occur.
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==Compatibility issues==
In the initial versions of [[SGML]] and [[HTML]], numeric character references were interpreted in relationship to the document character encoding, rather than [[Unicode]]. For Latin-script documents, numeric character references to characters between x80 and x9F in those documents will not be correct against [[Unicode]], and must be recoded. HTML standards prior to [[HTML 4]]
For example, as mentioned above, the correct numeric character reference for the [[Euro sign]] "€" <code>U+20AC</code> when using [[Unicode]] is decimal <code>&#8364;</code> and hexadecimal <code>&#x20AC;</code>. However, if using tools supporting obsolete implementations of HTML, the reference <code>&#128;</code> (Euro sign in the [[
As another example, if some text was created originally using the [[MacRoman]] character set, the [[quotation mark
==See also==
* [[List of XML and HTML character entity references]]
==References==
{{Reflist}}
{{Unicode navigation}}
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[[Category:Unicode]]
[[Category:XML]]
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