How to define a 'literal' in XSD - xsd

Question

I'm a newbie with XSD and pretty much at wits end.
What I need to do is somehow define a 'literal' element within an XSD file so that users can use it in an XML document.
For example:
In the XML document, I want to let a user be able to add an element like this:
<WordBoundary/>
but let the XSD file define not only the name 'WordBoundary' but also a list of Elements within which are of types defined elsewhere within the XSD.
Is this possible?
Updated to provide more information:
I am trying to write an XSD (or set of XSDs) which provide a library of pre-defined elements for an end-user to use.
The elements will ultimately be used to generate a Regular Expression but I want to hide the complexities of regular expressions from users as far as possible.
The XSD will use .NETs XSD.Exe to generate C# classes.
Currently I have defined elements like and and etc. which work fine but rely on their ultimate regex pattern being defined in code. Some of the elements have a Pattern attribute to allow fine tuning but this clutters up the XML document for users somewhat.
Examples of the built-up existing C# definitions:
const string BidMatch = #"(?<" + BidGroupName + ">" + DecimalNumberFragment + ")";
const string OfferMatch = #"(?<" + OfferGroupName + ">" + DecimalNumberFragment + ")";
const string BidOfferSpreadMatch = BidMatch + OptionalGap + RangeSeparatorFragment + OptionalGap + OfferMatch;
I therefore wanted to be able to refactor this so that all the regex patterns are moved from code into XSD definitions to form a library (or if it turns out not to be possible an XML file looked up by name) whereby there are primitives such as and and commonly-used but more complex structures such as etc.
So the use can use predefined elements like directly within their XML document but also be able to build up their own like this:
<Group captureName="MyCustomMatch">
<WordBoundary/>
<Digit/>
<Literal pattern="[xyz]" />
<AnyOf/>
<AnyOfChoice>
<DecimalNumber />
<Gap />
<DecimalNumber />
</AnyOfChoice>
<AnyOfChoice>
<LiteralText text="(" />
<DecimalNumber />
<LiteralText text=")" />
</AnyOfChoice>
<WordBoundary/>
</Group>
(In fact the prefined elements in the library would be build up the same way)
From the code point of view, I currently have AnyOf/Group etc. working by looking at the element type and calling a method to generate the pattern. For this to work, all these new Elements need to have a common ancestor, e.g. RegexLiteral, from which I can just read the Pattern attribute or whatever and add it into the full pattern.
I have tried extending a common type and trying to override its Pattern attribute to use a fixed attribute but XSD does not apparently allow this.
I am hoping this is just a limitation of my XSD knowledge rather than of XSD itself and hoping you clever guys know a way of achieving this.
Update2:
Thought I had it with this XSD fragment
<xs:complexType name="LiteralFragment" abstract="true">
<xs:attribute name="Pattern" type="xs:string" />
</xs:complexType>
<xs:complexType name="Fred">
<xs:complexContent >
<xs:extension base="LiteralFragment" >
<xs:attribute name="Pattern" type="xs:string" fixed="BBB" />
</xs:extension>
</xs:complexContent>
</xs:complexType>
but XSD (and Xsd2Code for that matter) generate this rubbish code:
public partial class Fred : LiteralFragment
{
private string pattern1Field;
public Fred()
{
this.pattern1Field = "BBB";
}
[System.Xml.Serialization.XmlAttributeAttribute("Pattern")]
public string Pattern1
{
get { return this.pattern1Field; }
set { this.pattern1Field = value; }
}
}
which bombs because there are two XmlAttributeAttribute with "Pattern" being used.
What I need is an XSD generator that is intelligent enough to realise this and generate this code instead:
public partial class Fred : LiteralFragment
{
public Fred()
{
Pattern = "BBB";
}
}

Answer 1

Try this....
The XSD:
<?xml version="1.0" encoding="utf-8"?>
<!--XML Schema generated by QTAssistant/XSD Module (http://www.paschidev.com)-->
<xsd:schema attributeFormDefault="unqualified" elementFormDefault="qualified" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<xsd:element name="root" type="TRoot"/>
<xsd:element name="WordBoundary">
<xsd:complexType/>
</xsd:element>
<xsd:element name="SomethingElse">
<xsd:complexType mixed="true">
<xsd:sequence>
<xsd:any minOccurs="0" maxOccurs="unbounded" processContents="lax"/>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
<xsd:complexType name="TRoot">
<xsd:sequence>
<xsd:any minOccurs="0" maxOccurs="unbounded" processContents="lax"/>
</xsd:sequence>
</xsd:complexType>
</xsd:schema>
And this XML:
<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<!-- Sample XML generated by QTAssistant (http://www.paschidev.com) -->
<root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<SomethingElse>Text <WordBoundary/> another <WordBoundary/>...</SomethingElse>
</root>
Is this what you had in mind?