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Schema Restrictions

Restrictions define acceptable values for elements and attributes. If a Schema restricts the value of an element to a certain length or certain value, the element cannot contain text that violates the restriction.

The most common types of restrictions include value restrictions, sets of values restrictions, and length restrictions. Restrictions are defined within the element definition. The code block below shows the general format for applying a restriction to an existing element.

Element Restrictions

<xsd:element name="element name">
    <xsd:simpleType>
        <xsd:restriction base="xsd: Integer">
            <xsd:minInclusive value="0"/>
            <xsd:maxInclusive value="10"/>
        </xsd:restriction>
    </xsd:simpleType>
</xsd:element>

Listing 3-14. Element restrictions.

Restrictions are defined within a <xsd:simpleType> block. Here the element is restricted to an integer value between 0 and 10 inclusive. minInclusive and maxInclusive are contstraints that define the possible values that can be assigned to the element. Other common contsraints are listed in the table below.

Schema Datatype Restrictions

Constraint Description
enumeration Defines a list of acceptable values
length Specifies the exact number of characters or list items allowed. Must be equal to or greater than zero
maxExclusive Specifies the upper bounds for numeric values (the value must be less than this value)
maxInclusive Specifies the upper bounds for numeric values (the value must be less than or equal to this value)
maxLength Specifies the maximum number of characters or list items allowed. Must be equal to or greater than zero
minExclusive Specifies the lower bounds for numeric values (the value must be greater than this value)
minInclusive Specifies the lower bounds for numeric values (the value must be greater than or equal to this value)
minLength Specifies the minimum number of characters or list items allowed. Must be equal to or greater than zero
totalDigits Specifies the exact number of digits allowed. Must be greater than zero
pattern Defines the exact sequence of characters that are acceptable

In the next example, the enumeration constraint is used to limit the values for the Size element. The values for the Size element are limited to small, medium, or large.

<xsd:element name="Size">
    <xsd:simpleType>
        <xsd:restriction base="xsd:string">
            <xsd:enumeration value="small"/>
            <xsd:enumeration value="medium"/
            <xsd:enumeration value="large"/>
        </xsd:restriction>
    </xsd:simpleType>
</xsd:element>

Listing 3-15. enumeration constraint.

The example below uses the pattern constraint to place restrictions on the id element. The restriction specifies that the value of the id element must be an integer and four digits in length. The first digit can be any number between 1 and 9, the remaining digits can be any number between 0 and 9.

<xsd:element name="id">
    <xsd:simpleType>
        <xsd:restriction base="xsd:integer">
            <xsd:pattern value="[1-9][0-9][0-9][0-9]"/>
        </xsd:restriction>
    </xsd:simpleType>
</xsd:element>

Listing 3-16. pattern constraint.


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