Basically, any time you use a named capturing group, it actually pushes the capture onto a named stack. You can then pop this stack by using the same capturing group prefixed with a hyphen, like (?<-stackToPop>).

Of course, anyone who finds themselves in this situation is going to ask: can it match XML?

It’s possible that I am missing something completely (it is rather late at night), but … very nearly. I haven’t quite figured out a nested section in the local DTD subset, but no one uses that feature anyway. (Can you spot it?)

Aside from that, most of the well-formedness criteria are handled (the obvious one being element nesting). Things that require non-local information such as entities aren’t handled. I think it is possible to handle duplicate attribute names in this form as well (via a lookahead for duplicate names).

Here is the code, and a test file which shows some stuff that is caught by this. Breaking any of the elements should make it fail:

var surrogate = @"([\ud800-\udbff][\udc00-\udfff])";// .NET can't handle \U10000-\u10FFFF
var c = @"([\u0009\u000a\u000d\u0020-\ud7ff\ue000-\ufffd]|"+surrogate + ")"; 
var s = @"([\u0020\u0009\u000d\u000a]+)";
var nameStartChar = @"([:A-Z_a-z\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02FF\u0370-\u037D\u037F-\u1FFF\u200C-\u200D\u2070-\u218F\u2C00-\u2FEF\u3001-\uD7FF\uF900-\uFDCF\uFDF0-\uFFFD]|" + surrogate + ")";
var nameChar = "(" + nameStartChar + @"|[-.0-9\u00B7\u0300-\u036F\u203F-\u2040])";
var name = "(?'name'" + nameStartChar + nameChar + "*)";
var names = "(?'names'" + name + @"(\u0020" + name +")*)";
var nmtoken = "(?'nmtoken'" + nameChar + "+)";
var nmtokens = "(?'nmtokens'" + nmtoken + @"(\u0020" + nmtoken +")*)";
var pereference = "%" + name + ";";
var entityReference= "(?'entityRef'&" + name + ";)";
var charref = @"&\#([0-9]+|x[0-9a-fA-F]+);";
var reference = "(?'reference'"+ entityReference + "|" + charref + ")";
var entityValue = "(?'entityValue'\"([^%&\"]|" + pereference + "|" + reference + ")*\"|'([^%&']|" + pereference + "|" + reference + ")*')";
var eq = "(?'eq'" + s + "?=" + s + "?)";
var versionNum  = @"1\.[0-9]+";
var comment = "(?'comment'<!--((?!--)" + c + ")*-->)";
var PITarget = "(?'pitarget'(?![xX][mM][lL])"+name+")";
var PI= @"(?'PI'<\?" + PITarget + "(" + s + @"((?!\?>)" + c + @")*)?\?>)";
var misc = "(?'misc'" + comment + "|" + PI + "|" + s + ")";
var versionInfo = "(?'versionInfo'"+ s + "version" + eq + "('" + versionNum + "'|\"" + versionNum + "\"))";
var encName = "(?'encName'[A-Za-z][A-Za-z0-9._-]*)";
var encodingDecl = "(?'encodingDecl'" + s + "encoding" + eq + "(\"" + encName + "\"|'"+ encName + "'))";
var sddecl = "(?'sddecl'" + s + "standalone" + eq + "(\"(yes|no)\"|'(yes|no)'))";
var xmlDecl = @"(?'xmlDecl'<\?xml" + versionInfo + encodingDecl + "?" + sddecl + "?" + s + @"?\?>)"; 
var mixed = @"(?'mixed'\(" + s + @"?\#PCDATA" + "(" + s + @"?\|" + s + "?" + name +")*" + s + "?" + @"\)\*|\(" +s + @"?\#PCDATA" + s + @"?\))";
var children = @"(?'children'unsureifpossible)";
var contentSpec = "(?'contentspec'EMPTY|ANY|"+mixed+"|"+children+")";
var elementDecl = "(?'elementdecl'<!ELEMENT" + s + name + s + contentSpec + s + "?>)";
var stringType = "CDATA";
var tokenizedType = "(ID(REF(S)?)?|ENTIT(Y|IES)|NMTOKENS?)";
var notationType = "(?'notation'NOTATION" +s + @"\(" + s + "?" + name + "(" + s + @"?\|" + s + "?" + name + ")*" + s + @"?\))";
var enumeration = @"(?'enumeration'\(" + s + "?" + nmtoken + "(" + s + @"?\|" + s + "?" + nmtoken + ")*" + s + @"?\))";
var enumeratedType = "(?'enumType'" + notationType + "|" + enumeration +")";
var attType = "(?'attType'" + stringType + "|" + tokenizedType + "|" + enumeratedType + ")";
var attValue = "(?'attValue'\"([^<&\"]|" +reference+ ")*\"|'([^<&']|" + reference + ")*')";
var defaultDecl = @"(?'defaultDecl'\#REQUIRED|\#IMPLIED|(\#FIXED"+s+")?" + attValue + ")";
var attDef = "(?'attDef'"+ s + name + s + attType + s + defaultDecl + ")";
var attListDecl = "(?'attlist'<!ATTLIST" + s + name + attDef + "*" + s + "?>)";
var systemLiteral = "(?'systemLiteral'\"[^\"]*\"|'[^']*')";
var pubIdChar = @"[a-zA-Z0-9'()+,./:=?;!*#@$_%\u0020\u000d\u000a-]";
var pubidLiteral = "(?'pubIdLiteral'\"" + pubIdChar + "*\"|'((?!')" + pubIdChar + ")*')";
var externalID = "(?'externalID'SYSTEM" + s + systemLiteral +"|PUBLIC" + s + pubidLiteral + s + systemLiteral+")";
var nDataDecl = "(?'ndatadecl'"+s + "NDATA" + s + name + ")";
var entityDef  = "(?'entityDef'" + entityValue + "|(" +externalID + nDataDecl + "?))";
var peDef = "(?'pedef'" + entityValue + "|"  + externalID + ")";
var GEDecl = "(?'gedecl'<!ENTITY" + s + name + s + entityDef + s + "?>)";
var PEDecl = "(?'gedecl'<!ENTITY" + s + "%" + s + name + s + peDef + s + "?>)";
var entityDecl = "(?'entityDecl'"+ GEDecl + "|" + PEDecl +")";
var publicID = "(?'publicID'PUBLIC" + s + pubidLiteral + ")";
var notationDecl = "(?'notationDecl'<!NOTATION" +  s + name + s + "(" + externalID + "|" + publicID + ")" + s + "?>)";
var markupDecl = "(?'markupdecl'" + elementDecl + "|" + attListDecl + "|" + entityDecl + "|" + notationDecl + "|" + PI + "|" + comment + ")";
var DeclSep = "(?'declSep'" + pereference + "|" + s + ")";
var intSubSet = @"(?'intSubSet'(" + markupDecl + "|" + DeclSep + ")*)"; 
var docTypeDecl = "(?'doctypedecl'<!DOCTYPE" + s + name + "(" + s + externalID+ ")?" + s + @"?(\[" + intSubSet + @"\]" + s + "?)?>)"; 
var prolog = xmlDecl + "?" + misc + "*(" + docTypeDecl + misc + "*)?"; 
var attribute = "(?'attribute'" +name + eq + attValue + ")";
var CDSect = @"(?'CDSect'<!\[CDATA\[((?!\]\]>)"+c+@")*\]\]>)";
var charData = @"(((?!\]\]>)[^<&])*)";
var content = @"(?>" + // minor optimization... don't backtrack over this (makes failing faster)
		@"<(?'openclose'" + name + @")(" + s + attribute + ")*" + s + @"?/>|"+
		@"<(?'open'"+ name +@")(" + s + attribute + ")*" + s + @"?>|"+
		@"</(?=\k'open'" + s + @"?>)(?'close-open'" + name + ")" + s +@"?>|"
		+reference+@"|"
		+PI+@"|"
		+comment+@"|"
		+CDSect+@"|"
		+charData+@")*" + 
	"(?(open)(?!))";
var rootElement = @"(?'root'(<(?'rootName'" + name + ")(" + s + attribute + ")*" + s + @"?>" + content + @"</\k'rootName'" + s + "?>)|(<(?'rootName'" + name + ")(" + s + attribute + ")*" + s + @"?/>))";
 
var document = "^" + prolog + rootElement + misc + "*" + "$";
 
var testDoc = @"<?xml version='1.0' encoding=""utf-8""?><!DOCTYPE nothtml []><items>
	<item available=""yes"" >
		<name> laptop  </name>
		<![CDATA[something14!$]] 1412]]>
		<"+"\U00010000"+@"quantity>  2 &amp; &#121; &#x234f; </"+"\U00010000"+@"quantity>
	</item><?notxml?>" /* or <?xml?> here */ +@"
	<item available=""yes"" x='' y=""&amp;"">
		<name> mouse </name >
		<quantity> 1 " + /* or ]]> invalid here */  @" </quantity>
	</item>
	<item available=""no"" >
		<!----> <!-- --> <!-- - -->" + /* or <!-- -- --> here */ @"
		<name> keyboad </name>
		<quantity> 0</quantity>
	</item>
</items><!-- stuff can go here --> <!-- yup --> <?pi aasd as!@*&$^!*@&$!@ ?>";
 
//Console.WriteLine(document);
Console.WriteLine(Regex.Match(testDoc, document, RegexOptions.IgnorePatternWhitespace|RegexOptions.Singleline|RegexOptions.ExplicitCapture));

And here’s the regex (with apologies to Mail::RFC822::Address):

^(?'xmlDecl'<\?xml(?'versionInfo'([\u0020\u0009\u000d\u000a]+)version(?'eq'([\u0
020\u0009\u000d\u000a]+)?=([\u0020\u0009\u000d\u000a]+)?)('1\.[0-9]+'|"1\.[0-9]+
"))(?'encodingDecl'([\u0020\u0009\u000d\u000a]+)encoding(?'eq'([\u0020\u0009\u00
0d\u000a]+)?=([\u0020\u0009\u000d\u000a]+)?)("(?'encName'[A-Za-z][A-Za-z0-9._-]*
)"|'(?'encName'[A-Za-z][A-Za-z0-9._-]*)'))?(?'sddecl'([\u0020\u0009\u000d\u000a]
+)standalone(?'eq'([\u0020\u0009\u000d\u000a]+)?=([\u0020\u0009\u000d\u000a]+)?)
("(yes|no)"|'(yes|no)'))?([\u0020\u0009\u000d\u000a]+)?\?>)?(?'misc'(?'comment'<
!--((?!--)([\u0009\u000a\u000d\u0020-\ud7ff\ue000-\ufffd]|([\ud800-\udbff][\udc0
0-\udfff])))*-->)|(?'PI'<\?(?'pitarget'(?![xX][mM][lL])(?'name'([:A-Z_a-z\u00C0-
\u00D6\u00D8-\u00F6\u00F8-\u02FF\u0370-\u037D\u037F-\u1FFF\u200C-\u200D\u2070-\u
218F\u2C00-\u2FEF\u3001-\uD7FF\uF900-\uFDCF\uFDF0-\uFFFD]|([\ud800-\udbff][\udc0
0-\udfff]))(([:A-Z_a-z\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02FF\u0370-\u037D\u037F
-\u1FFF\u200C-\u200D\u2070-\u218F\u2C00-\u2FEF\u3001-\uD7FF\uF900-\uFDCF\uFDF0-\
uFFFD]|([\ud800-\udbff][\udc00-\udfff]))|[-.0-9\u00B7\u0300-\u036F\u203F-\u2040]
)*))(([\u0020\u0009\u000d\u000a]+)((?!\?>)([\u0009\u000a\u000d\u0020-\ud7ff\ue00
0-\ufffd]|([\ud800-\udbff][\udc00-\udfff])))*)?\?>)|([\u0020\u0009\u000d\u000a]+
))*((?'doctypedecl'<!DOCTYPE([\u0020\u0009\u000d\u000a]+)(?'name'([:A-Z_a-z\u00C
0-\u00D6\u00D8-\u00F6\u00F8-\u02FF\u0370-\u037D\u037F-\u1FFF\u200C-\u200D\u2070-
\u218F\u2C00-\u2FEF\u3001-\uD7FF\uF900-\uFDCF\uFDF0-\uFFFD]|([\ud800-\udbff][\ud
c00-\udfff]))(([:A-Z_a-z\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02FF\u0370-\u037D\u03
7F-\u1FFF\u200C-\u200D\u2070-\u218F\u2C00-\u2FEF\u3001-\uD7FF\uF900-\uFDCF\uFDF0
-\uFFFD]|([\ud800-\udbff][\udc00-\udfff]))|[-.0-9\u00B7\u0300-\u036F\u203F-\u204
0])*)(([\u0020\u0009\u000d\u000a]+)(?'externalID'SYSTEM([\u0020\u0009\u000d\u000
a]+)(?'systemLiteral'"[^"]*"|'[^']*')|PUBLIC([\u0020\u0009\u000d\u000a]+)(?'pubI
dLiteral'"[a-zA-Z0-9'()+,./:=?;!*#@$_%\u0020\u000d\u000a-]*"|'((?!')[a-zA-Z0-9'(
)+,./:=?;!*#@$_%\u0020\u000d\u000a-])*')([\u0020\u0009\u000d\u000a]+)(?'systemLi
teral'"[^"]*"|'[^']*')))?([\u0020\u0009\u000d\u000a]+)?(\[(?'intSubSet'((?'marku
pdecl'(?'elementdecl'<!ELEMENT([\u0020\u0009\u000d\u000a]+)(?'name'([:A-Z_a-z\u0
0C0-\u00D6\u00D8-\u00F6\u00F8-\u02FF\u0370-\u037D\u037F-\u1FFF\u200C-\u200D\u207
0-\u218F\u2C00-\u2FEF\u3001-\uD7FF\uF900-\uFDCF\uFDF0-\uFFFD]|([\ud800-\udbff][\
udc00-\udfff]))(([:A-Z_a-z\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02FF\u0370-\u037D\u
037F-\u1FFF\u200C-\u200D\u2070-\u218F\u2C00-\u2FEF\u3001-\uD7FF\uF900-\uFDCF\uFD
F0-\uFFFD]|([\ud800-\udbff][\udc00-\udfff]))|[-.0-9\u00B7\u0300-\u036F\u203F-\u2
040])*)([\u0020\u0009\u000d\u000a]+)(?'contentspec'EMPTY|ANY|(?'mixed'\(([\u0020
\u0009\u000d\u000a]+)?\#PCDATA(([\u0020\u0009\u000d\u000a]+)?\|([\u0020\u0009\u0
00d\u000a]+)?(?'name'([:A-Z_a-z\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02FF\u0370-\u0
37D\u037F-\u1FFF\u200C-\u200D\u2070-\u218F\u2C00-\u2FEF\u3001-\uD7FF\uF900-\uFDC
F\uFDF0-\uFFFD]|([\ud800-\udbff][\udc00-\udfff]))(([:A-Z_a-z\u00C0-\u00D6\u00D8-
\u00F6\u00F8-\u02FF\u0370-\u037D\u037F-\u1FFF\u200C-\u200D\u2070-\u218F\u2C00-\u
2FEF\u3001-\uD7FF\uF900-\uFDCF\uFDF0-\uFFFD]|([\ud800-\udbff][\udc00-\udfff]))|[
-.0-9\u00B7\u0300-\u036F\u203F-\u2040])*))*([\u0020\u0009\u000d\u000a]+)?\)\*|\(
([\u0020\u0009\u000d\u000a]+)?\#PCDATA([\u0020\u0009\u000d\u000a]+)?\))|(?'child
ren'unsureifpossible))([\u0020\u0009\u000d\u000a]+)?>)|(?'attlist'<!ATTLIST([\u0
020\u0009\u000d\u000a]+)(?'name'([:A-Z_a-z\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02F
F\u0370-\u037D\u037F-\u1FFF\u200C-\u200D\u2070-\u218F\u2C00-\u2FEF\u3001-\uD7FF\
uF900-\uFDCF\uFDF0-\uFFFD]|([\ud800-\udbff][\udc00-\udfff]))(([:A-Z_a-z\u00C0-\u
00D6\u00D8-\u00F6\u00F8-\u02FF\u0370-\u037D\u037F-\u1FFF\u200C-\u200D\u2070-\u21
8F\u2C00-\u2FEF\u3001-\uD7FF\uF900-\uFDCF\uFDF0-\uFFFD]|([\ud800-\udbff][\udc00-
\udfff]))|[-.0-9\u00B7\u0300-\u036F\u203F-\u2040])*)(?'attDef'([\u0020\u0009\u00
0d\u000a]+)(?'name'([:A-Z_a-z\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02FF\u0370-\u037
D\u037F-\u1FFF\u200C-\u200D\u2070-\u218F\u2C00-\u2FEF\u3001-\uD7FF\uF900-\uFDCF\
uFDF0-\uFFFD]|([\ud800-\udbff][\udc00-\udfff]))(([:A-Z_a-z\u00C0-\u00D6\u00D8-\u
00F6\u00F8-\u02FF\u0370-\u037D\u037F-\u1FFF\u200C-\u200D\u2070-\u218F\u2C00-\u2F
EF\u3001-\uD7FF\uF900-\uFDCF\uFDF0-\uFFFD]|([\ud800-\udbff][\udc00-\udfff]))|[-.
0-9\u00B7\u0300-\u036F\u203F-\u2040])*)([\u0020\u0009\u000d\u000a]+)(?'attType'C
DATA|(ID(REF(S)?)?|ENTIT(Y|IES)|NMTOKENS?)|(?'enumType'(?'notation'NOTATION([\u0
020\u0009\u000d\u000a]+)\(([\u0020\u0009\u000d\u000a]+)?(?'name'([:A-Z_a-z\u00C0
-\u00D6\u00D8-\u00F6\u00F8-\u02FF\u0370-\u037D\u037F-\u1FFF\u200C-\u200D\u2070-\
u218F\u2C00-\u2FEF\u3001-\uD7FF\uF900-\uFDCF\uFDF0-\uFFFD]|([\ud800-\udbff][\udc
00-\udfff]))(([:A-Z_a-z\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02FF\u0370-\u037D\u037
F-\u1FFF\u200C-\u200D\u2070-\u218F\u2C00-\u2FEF\u3001-\uD7FF\uF900-\uFDCF\uFDF0-
\uFFFD]|([\ud800-\udbff][\udc00-\udfff]))|[-.0-9\u00B7\u0300-\u036F\u203F-\u2040
])*)(([\u0020\u0009\u000d\u000a]+)?\|([\u0020\u0009\u000d\u000a]+)?(?'name'([:A-
Z_a-z\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02FF\u0370-\u037D\u037F-\u1FFF\u200C-\u2
00D\u2070-\u218F\u2C00-\u2FEF\u3001-\uD7FF\uF900-\uFDCF\uFDF0-\uFFFD]|([\ud800-\
udbff][\udc00-\udfff]))(([:A-Z_a-z\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02FF\u0370-
\u037D\u037F-\u1FFF\u200C-\u200D\u2070-\u218F\u2C00-\u2FEF\u3001-\uD7FF\uF900-\u
FDCF\uFDF0-\uFFFD]|([\ud800-\udbff][\udc00-\udfff]))|[-.0-9\u00B7\u0300-\u036F\u
203F-\u2040])*))*([\u0020\u0009\u000d\u000a]+)?\))|(?'enumeration'\(([\u0020\u00
09\u000d\u000a]+)?(?'nmtoken'(([:A-Z_a-z\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02FF\
u0370-\u037D\u037F-\u1FFF\u200C-\u200D\u2070-\u218F\u2C00-\u2FEF\u3001-\uD7FF\uF
900-\uFDCF\uFDF0-\uFFFD]|([\ud800-\udbff][\udc00-\udfff]))|[-.0-9\u00B7\u0300-\u
036F\u203F-\u2040])+)(([\u0020\u0009\u000d\u000a]+)?\|([\u0020\u0009\u000d\u000a
]+)?(?'nmtoken'(([:A-Z_a-z\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02FF\u0370-\u037D\u
037F-\u1FFF\u200C-\u200D\u2070-\u218F\u2C00-\u2FEF\u3001-\uD7FF\uF900-\uFDCF\uFD
F0-\uFFFD]|([\ud800-\udbff][\udc00-\udfff]))|[-.0-9\u00B7\u0300-\u036F\u203F-\u2
040])+))*([\u0020\u0009\u000d\u000a]+)?\))))([\u0020\u0009\u000d\u000a]+)(?'defa
ultDecl'\#REQUIRED|\#IMPLIED|(\#FIXED([\u0020\u0009\u000d\u000a]+))?(?'attValue'
"([^<&"]|(?'reference'(?'entityRef'&(?'name'([:A-Z_a-z\u00C0-\u00D6\u00D8-\u00F6
\u00F8-\u02FF\u0370-\u037D\u037F-\u1FFF\u200C-\u200D\u2070-\u218F\u2C00-\u2FEF\u
3001-\uD7FF\uF900-\uFDCF\uFDF0-\uFFFD]|([\ud800-\udbff][\udc00-\udfff]))(([:A-Z_
a-z\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02FF\u0370-\u037D\u037F-\u1FFF\u200C-\u200
D\u2070-\u218F\u2C00-\u2FEF\u3001-\uD7FF\uF900-\uFDCF\uFDF0-\uFFFD]|([\ud800-\ud
bff][\udc00-\udfff]))|[-.0-9\u00B7\u0300-\u036F\u203F-\u2040])*);())|&\#([0-9]+|
x[0-9a-fA-F]+);()))*"|'([^<&']|(?'reference'(?'entityRef'&(?'name'([:A-Z_a-z\u00
C0-\u00D6\u00D8-\u00F6\u00F8-\u02FF\u0370-\u037D\u037F-\u1FFF\u200C-\u200D\u2070
-\u218F\u2C00-\u2FEF\u3001-\uD7FF\uF900-\uFDCF\uFDF0-\uFFFD]|([\ud800-\udbff][\u
dc00-\udfff]))(([:A-Z_a-z\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02FF\u0370-\u037D\u0
37F-\u1FFF\u200C-\u200D\u2070-\u218F\u2C00-\u2FEF\u3001-\uD7FF\uF900-\uFDCF\uFDF
0-\uFFFD]|([\ud800-\udbff][\udc00-\udfff]))|[-.0-9\u00B7\u0300-\u036F\u203F-\u20
40])*);())|&\#([0-9]+|x[0-9a-fA-F]+);()))*')))*([\u0020\u0009\u000d\u000a]+)?>)|
(?'entityDecl'(?'gedecl'<!ENTITY([\u0020\u0009\u000d\u000a]+)(?'name'([:A-Z_a-z\
u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02FF\u0370-\u037D\u037F-\u1FFF\u200C-\u200D\u2
070-\u218F\u2C00-\u2FEF\u3001-\uD7FF\uF900-\uFDCF\uFDF0-\uFFFD]|([\ud800-\udbff]
[\udc00-\udfff]))(([:A-Z_a-z\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02FF\u0370-\u037D
\u037F-\u1FFF\u200C-\u200D\u2070-\u218F\u2C00-\u2FEF\u3001-\uD7FF\uF900-\uFDCF\u
FDF0-\uFFFD]|([\ud800-\udbff][\udc00-\udfff]))|[-.0-9\u00B7\u0300-\u036F\u203F-\
u2040])*)([\u0020\u0009\u000d\u000a]+)(?'entityDef'(?'entityValue'"([^%&"]|%(?'n
ame'([:A-Z_a-z\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02FF\u0370-\u037D\u037F-\u1FFF\
u200C-\u200D\u2070-\u218F\u2C00-\u2FEF\u3001-\uD7FF\uF900-\uFDCF\uFDF0-\uFFFD]|(
[\ud800-\udbff][\udc00-\udfff]))(([:A-Z_a-z\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02
FF\u0370-\u037D\u037F-\u1FFF\u200C-\u200D\u2070-\u218F\u2C00-\u2FEF\u3001-\uD7FF
\uF900-\uFDCF\uFDF0-\uFFFD]|([\ud800-\udbff][\udc00-\udfff]))|[-.0-9\u00B7\u0300
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First of all we have to have an idea of what the numbers look like. We want to simulate this in XSL:

data Nat = Zero | Succ Nat

Using XSD we can have something like this:

<complexType name="Nat">
    <choice>
        <element name="Zero"/>
        <element name="Succ" type="Nat"/>
    </choice>
</complexType>

(Note that this is not tested, and probably wrong!)

However, since I don’t have an XSLT processor on hand that can deal with XSD types, I had to drop this course of action (although I still used the introduced type for the data). The type gives data like this:

<Succ><Succ><Zero/></Succ></Succ>

…in this case standing for the number two. Now the idea of addition on these numbers is given by a function:

add Zero y = y
add (Succ x) y = Succ (add x y)

Since this function involves pattern-matching it is an ideal candidate for implementation in XSLT, and it looks something like this:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="2.0" 
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
 
	<!-- apply the add function to two numbers -->
	<xsl:template name="add" match="add">
		<xsl:apply-templates mode="adder" select="*[1]">
			<xsl:with-param name="rest" select="*[2]"/>
		</xsl:apply-templates>
	</xsl:template>
 
	<!-- add Succ(x) y = Succ(add x y) -->
	<xsl:template name="addSucc" mode="adder" match="Succ">
		<xsl:param name="rest"/>
		<Succ>
			<xsl:apply-templates mode="adder">
				<xsl:with-param name="rest" select="$rest"/>
			</xsl:apply-templates>
		</Succ>
	</xsl:template>
 
	<!-- add Zero y = y -->
	<xsl:template name="addZero" mode="adder" match="Zero">
		<xsl:param name="rest"/>
		<xsl:copy-of select="$rest"/>
	</xsl:template>
 
</xsl:stylesheet>

The first template matches any occurances of <add/>, and calls the real ‘add’ function on them. It passes the second number as a parameter, while the first is what the function actually recurses on—exactly as in the Haskell example. The rest proceeds similarly.

You can test it yourself by using the W3C online XSLT processor, which uses my processor and some sample input data. It should work for any similar data that you can throw at it as well. In this case it is adding 2 + 2 and coming up with 4, as we expect

XML 1.1 allows you to insert characters from the C0 range as long as they are escaped as character entity references, and mandates that you do the same for those from the C1 range.

This little fact is the reason why not all XML 1.0-valid documents are valid under XML 1.1. Nasty.