A dandy little technique for constraining your strings to ASCII

Hi Folks,

So, you’ve created an XML schema. And it contains a lot of elements and attributes of type string.

You want each string constrained to just ASCII characters. Use the pattern facet for that.

Here’s a dandy little technique you can use:

At the top of your schema, place this named entity declaration:

<!DOCTYPE xs:schema [
<!ENTITY ASCII "[\p{IsBasicLatin}]*">
]>

The entity ( ASCII ) can then be referenced in each pattern facet:

<xs:simpleType name="NameType">
    <xs:restriction base="xs:string">
        <xs:maxLength value="10" />
        <xs:pattern value="&ASCII;" />
    </xs:restriction>
</xs:simpleType>

<xs:simpleType name="DescriptionType">
    <xs:restriction base="xs:string">
        <xs:maxLength value="20" />
        <xs:pattern value="&ASCII;" />
    </xs:restriction>
</xs:simpleType>

At parse-time the XML parser will substitute each entity reference ( &ASCII; ) with its replacement text ( [\p{IsBasicLatin}]* ).

The entity provides useful documentation; i.e., I assert that this:

<xs:pattern value="&ASCII;" />

is more readable than this:

<xs:pattern value="[\p{IsBasicLatin}]*" />

Here’s a complete schema to illustrate the technique:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE xs:schema [
<!ENTITY ASCII "[\p{IsBasicLatin}]*">
]>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
    
    <xs:element name="Test">
        <xs:complexType>
            <xs:sequence>
                <xs:element name="Name" type="NameType" />
                <xs:element name="Description" type="DescriptionType" />
            </xs:sequence>
        </xs:complexType>
    </xs:element>
   
    <xs:simpleType name="NameType">
        <xs:restriction base="xs:string">
            <xs:maxLength value="10" />
            <xs:pattern value="&ASCII;" />
        </xs:restriction>
    </xs:simpleType>
   
    <xs:simpleType name="DescriptionType">
        <xs:restriction base="xs:string">
            <xs:maxLength value="20" />
            <xs:pattern value="&ASCII;" />
        </xs:restriction>
    </xs:simpleType>

</xs:schema>

/Roger