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Sams Teach Yourself XML in 21 Days

Sep 16, 2005

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Restricting Simple Types by Using XML Schema Facets

One of the most important ways that XML schemas differ from DTDs is that they let you specify data types, such as strings and integers. As you saw yesterday, to do that, XML schemas let you use the data types that are built in to the XML schema specification, such as xsd:string, xsd:integer, and xsd:date. But you can do even more with data types—you can also restrict the values that are acceptable.

For example, take a look at the attribute named loanNumber, which is declared to be of type loanNumberType:

<xsd:complexType name="mortgagesType">
    <xsd:sequence>
        <xsd:element name="mortgage" minOccurs="1" maxOccurs="8">
            <xsd:complexType>
                <xsd:sequence>
                    <xsd:element name="property" type="xsd:string"/>
                    <xsd:element name="date" type="xsd:date" minOccurs="0"/>

                    <xsd:element name="loanAmount" type="xsd:decimal"/>
                    <xsd:element name="term">
                        <xsd:simpleType>
                            <xsd:restriction base="xsd:integer">
                                <xsd:maxInclusive value="30"/>
                            </xsd:restriction>
                        </xsd:simpleType>
                    </xsd:element>
                </xsd:sequence>
                <xsd:attribute name="loanNumber" type="loanNumberType"/>
            </xsd:complexType>
        </xsd:element>
    </xsd:sequence>
</xsd:complexType>

The attribute loanNumber is not of a predefined type; it's of the loanNumberType type, which we've defined ourselves. In particular, we've defined it with the <simpleType> element, like this:

<xsd:simpleType name="loanNumberType">
    <xsd:restriction base="xsd:string">
        <xsd:pattern value="\d{2} \d{4} \d{2}"/>
    </xsd:restriction>
</xsd:simpleType>

As you saw yesterday, you must base your own simple types on the simple types that are built into XML schemas. In this example, you use the xsd:string type. What's interesting here is that you can restrict the possible values of a simple type by using the <xsd:restriction> element. Here's what's happening: You're using the <xsd:restriction> element's base attribute to indicate that you are basing this type on the xsd:string type. Then you're using an XML schema facet to restrict the actual text that can be stored in attributes of the type you're creating.

XML schema facets let you restrict the data that a simple type can hold. In this example, the text in the loanNumberType type must match the regular expression pattern "\d{2} \d{4} \d{2}", which matches text strings made up of two digits, a space, four digits, another space, and two more digits (for example, "22 6666 99"). You don't have to know how to use regular expressions in this book, but this example shows how powerful facets can be. In this case, you're using the pattern facet to specify a regular expression pattern that text used for the loanNumber attribute must match: <xsd:pattern value="\d{2} \d{4} \d{2}"/>. At this point you've gone far beyond DTDs, which can't even specify data types.

There are simpler facets than the pattern facet. Two popular facets are the minInclusive and maxInclusive facets, which let you put lower and upper bounds on numeric values. For example, say that you want to create an attribute named dayNumber that holds the day of the year and can range from 1 to 366 (to allow for leap years). You might restrict the possible values that dayNumber can hold by making it of the type dayNumberType, which you can create by using <xsd:simpleType> and the minInclusive and maxInclusive facets, like this:

<xsd:simpleType name="dayNumber">
    <xsd:restriction base="xsd:integer">
        <xsd:minInclusive value="1"/>
        <xsd:maxInclusive value="366"/>
    </xsd:restriction>
</xsd:simpleType>

After you declare this new simple type, you can declare elements and attributes of this type.

The following are the available facets and how they constrain data values:

totalDigits — Specifies the maximum number of digits.
fractionDigits — Specifies the maximum number of decimal digits.
pattern — Specifies a regular expression that text must match.
whiteSpace — Can be set to preserve (to preserve white space), replace (to replace all white space with ), or collapse (to collapse multiple contiguous whitespace to one ).
enumeration — Constrains possible values to a specified set.
maxInclusive — Specifies the maximum possible value, inclusive.
maxExclusive — Specifies the maximum possible value, exclusive.
minInclusive — Specifies the minimum possible value, inclusive.
minExclusive — Specifies the minimum possible value, exclusive.
length — Specifies the data's length, such as number of characters.
minLength — Specifies the minimum possible length.
maxLength — Specifies the maximum possible length.

Which facets apply to which predefined simple types? Can you use a pattern facet with an xsd:int value, for example? Table 7.1 lists what facets you can use with the various simple data types. The numeric simple types and simple types that can be ordered also have some additional facets, as listed in Table 7.2.

Table 7.1. The Facets Available for Simple Types

Type	`length`	`minLength`	`maxLength`	`pattern`	`enumeration`	`whiteSpace`
`anyURI`	x	x	x	x	x	x
`base64Binary`	x	x	x	x	x	x
`boolean`				x		x
`byte`				x	x	x
`date`				x	x	x
`dateTime`				x	x	x
`decimal`				x	x	x
`double`				x	x	x
`duration`				x	x	x
`ENTITIES`	x	x	x		x	x
`ENTITX`	x	x	x	x	x	x
`float`				x	x	x
`gDay`				x	x	x
`gMonth`				x	x	x
`gMonthDay`				x	x	x
`gYear`				x	x	x
`gYearMonth`				x	x	x
`hexBinary`	x	x	x	x	x	x
`ID`	x	x	x	x	x	x
`IDREF`	x	x	x	x	x	x
`IDREFS`	x	x	x		x	x
`int`				x	x	x
`integer`				x	x	x
`language`	x	x	x	x	x	x
`long`				x	x	x
`Name`	x	x	x	x	x	x
`NCName`	x	x	x	x	x	x
`negativeInteger`				x	x	x
`NMTOKEN`	x	x	x	x	x	x
`NMTOKENS`	x	x	x		x	x
`nonNegativeInteger`				x	x	x
`nonPositiveInteger`				x	x	x
`normalizedString`	x	x	x	x	x	x
`NOTATION`	x	x	x	x	x	x
`positiveInteger`				x	x	x
`QName`	x	x	x	x	x	x
`short`				x	x	x
`string`	x	x	x	x	x	x
`time`				x	x	x
`token`	x	x	x	x	x	x
`unsignedByte`				x	x	x
`unsignedInt`				x	x	x
`unsignedLong`				x	x	x
`unsignedShort`				x	x	x

Table 7.2. The Facets Available for Simple Ordered Types

Type	`max-Inclusive`	`max-Exclusive`	`min-Inclusive`	`min-Exclusive`	`total-Digits`	`fraction-Digits`
`byte`	x	x	x	x	x	x
`unsignedByte`	x	x	x	x	x	x
`integer`	x	x	x	x	x	x
`positiveInteger`	x	x	x	x	x	x
`negativeInteger`	x	x	x	x	x	x
`nonNegativeInteger`	x	x	x	x	x	x
`nonPositiveInteger`	x	x	x	x	x	x
`int`	x	x	x	x	x	x
`unsignedInt`	x	x	x	x	x	x
`long`	x	x	x	x	x	x
`unsignedLong`	x	x	x	x	x	x
`short`	x	x	x	x	x	x
`unsignedShort`	x	x	x	x	x	x
`decimal`	x	x	x	x	x	x
`float`	x	x	x	x
`double`	x	x	x	x
`time`	x	x	x	x
`dateTime`	x	x	x	x
`duration`	x	x	x	x
`date`	x	x	x	x
`gMonth`	x	x	x	x
`gYear`	x	x	x	x
`gYearMonth`	x	x	x	x
`gDay`	x	x	x	x
`gMonthDay`	x	x	x	x

One of the interesting facets in these tables is enumeration, which lets you specify a set of values that a data item can select from. For example, to set up a simple type named dayOfTheWeek, whose values can be "Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", and "Saturday", you would define the type like this:

<xsd:simpleType name="dayOfTheWeek">
    <xsd:restriction base="xsd:string">
        <xsd:enumeration value="Sunday"/>
        <xsd:enumeration value="Monday"/>
        <xsd:enumeration value="Tuesday"/>
        <xsd:enumeration value="Wednesday"/>
        <xsd:enumeration value="Thursday"/>
        <xsd:enumeration value="Friday"/>
        <xsd:enumeration value="Saturday"/>
    </xsd:restriction>
 </xsd:simpleType>

NOTE

The extended power of facets, which let you specify and limit the values your data can legally have, is good for the XML author but not so good for programmers who want to support XML schemas. Supporting XML schemas is one thing, but, for example, supporting full regular expressions so you can use facets such as pattern is a very difficult thing to do for most programmers. And that's one of the reasons there aren't more validators that use XML schemas and why the implementation of validators that use XML schemas is only partial. Internet Explorer, for example, does not support the pattern facet. On the other hand, the Visual Studio .NET programming languages, such as Visual Basic .NET and Visual C# .NET, do support regular expressions, and so they support the pattern facet.

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