object it = new SomeStruct { Item = 1 };
 
	Floatsy(it);
 
	Console.WriteLine(((SomeStruct)it).Item);

At runtime, it will throw an InvalidCastException!

Here’s how. In .NET, each object has also associated with it a value which determines the type of the object. In memory, this is stored before the object’s data, like so (I got this information from the MSDN article Drill Into .NET Framework Internals to See How the CLR Creates Runtime Objects):

Now, if we can write to that, we can set the type of the object to whatever we want!

There’s one small problem — in .NET we can’t take the address of a managed object (which we need in order to write to the object in memory). There are various reasons for this, one of them being that the garbage collector likes to be able to move objects around. Being able to take arbitrary pointers of objects would mean that the pointers could become invalidated.

What we can do is to pin a struct, which allows us to retrieve the address of it (this facility exists so that users can pass pinned managed structs into unmanaged code as pointers). Here’s how to pin a struct:

    var handle = GCHandle.Alloc(o, GCHandleType.Pinned);
    var addr = handle.AddrOfPinnedObject();

We can then write to the type handle like so (both the sync block and type handle are 32 bits):

    *(((int*)addr)-1) = someValue;

Another problem is that all of this is only true for values on the heap. As far as I can tell, the static type of the variable is what .NET uses to identify values on the stack. So in order for this to work, we must use a boxed copy of a struct.

Finally, here’s some demo code, showing runtime changing of types for both a primitive and custom struct:

using System;
using System.Runtime.InteropServices;
 
unsafe class Program
{
    struct SomeStruct { public int Item; }
 
    public static void Main()
    {
        object notAFloat = 1;
        object me = new SomeStruct {Item = 1};
 
        Console.WriteLine("{0} ({1})", notAFloat, notAFloat.GetType());
        Floatify(notAFloat);
        Console.WriteLine("{0} ({1})", notAFloat, notAFloat.GetType());
        Console.WriteLine();
        Console.WriteLine(me);
        Floatify(me);
        Console.WriteLine(me);
    }
 
    static void Floatify<T>(T o)
    {
        var handle = GCHandle.Alloc(o, GCHandleType.Pinned);
        var addr = handle.AddrOfPinnedObject();
 
        object f = (float)1.0;
        var handle2 = GCHandle.Alloc(f, GCHandleType.Pinned);
        var addr2 = handle2.AddrOfPinnedObject();
 
        // copy type handle of a float to the object
        *(((int*)addr)-1) = *(((int*)addr2)-1);
 
        handle2.Free();
        handle.Free();
    }
}

The output is:

1 (System.Int32)
1.401298E-45 (System.Single)

Program+SomeStruct
1.401298E-45

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
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9\u000a\u000d\u0020-\ud7ff\ue000-\ufffd]|([\ud800-\udbff][\udc00-\udfff])))*\]\]
>)|(((?!\]\]>)[^<&])*))*(?(open)(?!))</\k'rootName'([\u0020\u0009\u000d\u000a]+)
?>)|(<(?'rootName'(?'name'([:A-Z_a-z\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02FF\u037
0-\u037D\u037F-\u1FFF\u200C-\u200D\u2070-\u218F\u2C00-\u2FEF\u3001-\uD7FF\uF900-
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attribute'(?'name'([:A-Z_a-z\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02FF\u0370-\u037D
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FDF0-\uFFFD]|([\ud800-\udbff][\udc00-\udfff]))(([:A-Z_a-z\u00C0-\u00D6\u00D8-\u0
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020\u0009\u000d\u000a]+)?)(?'attValue'"([^<&"]|(?'reference'(?'entityRef'&(?'nam
e'([:A-Z_a-z\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02FF\u0370-\u037D\u037F-\u1FFF\u2
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u036F\u203F-\u2040])*);())|&\#([0-9]+|x[0-9a-fA-F]+);()))*"|'([^<&']|(?'referenc
e'(?'entityRef'&(?'name'([:A-Z_a-z\u00C0-\u00D6\u00D8-\u00F6\u00F8-\u02FF\u0370-
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FDCF\uFDF0-\uFFFD]|([\ud800-\udbff][\udc00-\udfff]))(([:A-Z_a-z\u00C0-\u00D6\u00
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')))*([\u0020\u0009\u000d\u000a]+)?/>))(?'misc'(?'comment'<!--((?!--)([\u0009\u0
00a\u000d\u0020-\ud7ff\ue000-\ufffd]|([\ud800-\udbff][\udc00-\udfff])))*-->)|(?'
PI'<\?(?'pitarget'(?![xX][mM][lL])(?'name'([:A-Z_a-z\u00C0-\u00D6\u00D8-\u00F6\u
00F8-\u02FF\u0370-\u037D\u037F-\u1FFF\u200C-\u200D\u2070-\u218F\u2C00-\u2FEF\u30
01-\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\uFDF0-\uFFFD]|([\ud800-\udbf
f][\udc00-\udfff]))|[-.0-9\u00B7\u0300-\u036F\u203F-\u2040])*))(([\u0020\u0009\u
000d\u000a]+)((?!\?>)([\u0009\u000a\u000d\u0020-\ud7ff\ue000-\ufffd]|([\ud800-\u
dbff][\udc00-\udfff])))*)?\?>)|([\u0020\u0009\u000d\u000a]+))*$

That said, the vectorization code is clearly an improvement on mine, so let’s take that ball and run with it!

The Code

Now we use SIMD instructions to vectorize the counting of characters. I modified this from Colin’s routine, and I’m sure he has some bit-fiddling up his sleeves that would make this run even faster

As it is, I used a straightforward algorithm to extract the information.

#define GetMask(x) __builtin_ia32_pmovmskb128(x)
#define LoadBytes(x) __builtin_ia32_loaddqu(x)
#define CompareEquality(x,y) __builtin_ia32_pcmpeqb128((x),(y))
#define Or(x,y) __builtin_ia32_por128((x),(y))
#define NotExpected(x) __builtin_expect((x),0)
#define And(x,y) __builtin_ia32_pand128((x),(y))
 
typedef unsigned char v16qi __attribute__ ((vector_size(16)));
 
const char mask[16] = {
    0xc0, 0xc0, 0xc0, 0xc0,
    0xc0, 0xc0, 0xc0, 0xc0,
    0xc0, 0xc0, 0xc0, 0xc0,
    0xc0, 0xc0, 0xc0, 0xc0
};
const char match[16] = {
    0x80, 0x80, 0x80, 0x80,
    0x80, 0x80, 0x80, 0x80,
    0x80, 0x80, 0x80, 0x80,
    0x80, 0x80, 0x80, 0x80
};
const char zero[16] = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
unsigned char HammingWeight[65536]; //initialized elsewhere
 
static size_t cp_strlen_utf8_sse2(const char *_s)
{
    const char *s;
    const v16qi allZero = LoadBytes(zero);
    const v16qi masking = LoadBytes(mask);
    const v16qi matching = LoadBytes(match);
 
    v16qi row;
    size_t count = 0;
    unsigned char b;
 
    // unaligned bytes
    for (s = _s; (uintptr_t) (s) & (sizeof(v16qi) - 1); s++) {
        b = *s;
        if (b == '\0')
            goto done;
        count += (b >> 7) & ((~b) >> 6);
    }
 
    /* Handle complete blocks. */
    for (;; s += sizeof(v16qi)) {
        /* Prefetch */
        __builtin_prefetch(&s[256], 0, 0);
 
        /* Load Bytes */
        row = LoadBytes(s);
 
        /* Expect this to be false :) */
        if (NotExpected(GetMask(
                                   /* Check for zero bytes */
                                      CompareEquality(allZero, row))))
            break;
 
        /* Count number of non-starter bytes */
 
        row = And(row, masking);
        row = CompareEquality(row, matching);
        count += HammingWeight[GetMask(row)];
    }
 
    //leftover bytes
    for (;; s++) {
        b = *s;
        if (b == '\0')
            break;
        count += (b >> 7) & ((~b) >> 6);
    }
 
  done:
    return ((s - _s) - count);
}

Results

This counts about twice as fast as GCC/libc’s standard, non-UTF-8 strlen. Note the discrepancies between my timings of Colin’s code and his own tests. Damn thou, 32-bits!

"": 0 0 0 0 0 0 0
"hello, world": 12 12 12 12 12 12 12
"naïve": 6 6 6 5 5 5 5
"こんにちは": 15 15 15 5 5 5 5
"abcdefghijklmnopqrstuvwxyzβ": 28 28 28 27 27 27 27
testing 33554424 bytes of repeated "hello, world":
                      gcc_strlen =   33554424: 0.019331 +/- 0.001076
                      kjs_strlen =   33554424: 0.035095 +/- 0.000530
                       cp_strlen =   33554424: 0.021472 +/- 0.000310
                 kjs_strlen_utf8 =   33554424: 0.070260 +/- 0.000240
                  gp_strlen_utf8 =   33554424: 0.035144 +/- 0.000471
                  cp_strlen_utf8 =   33554424: 0.050539 +/- 0.000342
             cp_strlen_utf8_sse2 =   33554424: 0.010297 +/- 0.001551
testing 33554430 bytes of repeated "naïve":
                      gcc_strlen =   33554430: 0.019176 +/- 0.000824
                      kjs_strlen =   33554430: 0.035090 +/- 0.000478
                       cp_strlen =   33554430: 0.021472 +/- 0.000323
                 kjs_strlen_utf8 =   27962025: 0.070347 +/- 0.000354
                  gp_strlen_utf8 =   27962025: 0.054802 +/- 0.000299
                  cp_strlen_utf8 =   27962025: 0.050595 +/- 0.000602
             cp_strlen_utf8_sse2 =   27962025: 0.010011 +/- 0.001453
testing 33554430 bytes of repeated "こんにちは":
                      gcc_strlen =   33554430: 0.019331 +/- 0.000836
                      kjs_strlen =   33554430: 0.035225 +/- 0.000411
                       cp_strlen =   33554430: 0.021429 +/- 0.000309
                 kjs_strlen_utf8 =   11184810: 0.070249 +/- 0.000312
                  gp_strlen_utf8 =   11184810: 0.026545 +/- 0.000621
                  cp_strlen_utf8 =   11184810: 0.050512 +/- 0.000273
             cp_strlen_utf8_sse2 =   11184810: 0.010246 +/- 0.001466
testing 33554416 bytes of repeated "abcdefghijklmnopqrstuvwxyzβ":
                      gcc_strlen =   33554416: 0.019308 +/- 0.001091
                      kjs_strlen =   33554416: 0.035070 +/- 0.000486
                       cp_strlen =   33554416: 0.021441 +/- 0.000289
                 kjs_strlen_utf8 =   32356044: 0.070287 +/- 0.000297
                  gp_strlen_utf8 =   32356044: 0.043681 +/- 0.000429
                  cp_strlen_utf8 =   32356044: 0.050402 +/- 0.000204
             cp_strlen_utf8_sse2 =   32356044: 0.010407 +/- 0.001371