RE: [xml-dev] patterns and restrictions

David Tolpin wrote:
> 
> [ Charset ISO-8859-1 unsupported, converting... ]
> > Hi,
> > 
> > Does the following allow 'abc' as a valid value?
> > 
> > <xs:simpleType name="MyDouble">
> >         <xs:restriction base="xs:double">
> >             <xs:pattern value="[^N].*"/>
> >         </xs:restriction>
> > </xs:simpleType>
> > 
> > We had been working under the belief that, via restriction 
> - patterns
> > from both the new datatype and the original one are 'And'ed 
> together. 
> > i.e - the above datatype would only allow values valid for 
> a double - 
> > with the exception of NaN.
> > 
> > However, we've read somewhere today that whenever a pattern facet is
> > evaluated - it is evaluated against a string.  Though 
> XMLSpy and Xerces 
> 
> The 'XML Schema Part 2: Datatypes' says
> 
> NOTE: It is a consequence of the schema representation 
> constraint Multiple patterns (?4.3.4.3) and of the rules for 
> žrestrictionž that žpatternž facets specified on the same 
> step in a type derivation are ORed together, while žpatternž 
> facets specified on different steps of a type derivation are 
> ANDed together.
> 
> 'pattern' is a constraining facet, and a type defined by 
> applying a constraining facet to a primitive type is a derived type. 
> 
> Therefore, I would think that 'pattern' defines a subset of 
> the lexical space of the type it is applied to.


My understanding is that the "pattern" facet is a very special facet, in
that it constrains two different things:

1) the value space of the type;

2) the lexical representation of the values in the value space resulting
from 1, possibly beyond the extent implied by 1.

The text says that "pattern" constrains the value space "by constraining the
lexical space" (4.3.4), but also says that the literals must match the
pattern regexp (4.3.4.4).  Thus both (1) and (2) follow.

For example, in the very simple case of a pattern value "01" applied to
xsd:integer, the pattern facet has the following effects:

1) it restricts the value space of the datatype (derived from xsd:integer)
to include only the integer value 1;

2) it requires that the integer value 1 be lexically represented precisely
as "01", and not, say, as "1" or "001".

"Normal" facets (except "pattern") don't care about the lexical
representation.  Any lexical form that is legal for the values in the value
space may be used in instances, and will "represent" one of those values.
"Pattern" is different.

Alessandro Triglia


> 
> David Tolpin
> 
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Received on Wednesday, 17 December 2003 11:43:29 UTC