11. XML Data Types in XSD

What you will learn

• The built-in data types you use most
• How to restrict a type with facets
• How to make your own named simple type
• How to allow only certain values, or a pattern

Why data types?

To a plain XML parser, <age>25</age> and <age>twenty</age> are both just text. A schema can say that age must be a whole number, so the second one is rejected before your program ever sees it.

Common built-in types

Dates follow the international year-month-day order, with two digits for the month and day.

Using types

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <xs:element name="order">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="customer" type="xs:string"/>
        <xs:element name="quantity" type="xs:positiveInteger"/>
        <xs:element name="total" type="xs:decimal"/>
        <xs:element name="gift" type="xs:boolean"/>
        <xs:element name="date" type="xs:date"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

</xs:schema>

<order>
  <customer>Sam</customer>
  <quantity>3</quantity>
  <total>37.50</total>
  <gift>true</gift>
  <date>2026-03-15</date>
</order>

Restrictions and facets

You can narrow a type with a restriction. Inside it, facets set the limits.

A number in a range

<xs:element name="age">
  <xs:simpleType>
    <xs:restriction base="xs:integer">
      <xs:minInclusive value="13"/>
      <xs:maxInclusive value="120"/>
    </xs:restriction>
  </xs:simpleType>
</xs:element>

A list of allowed values

<xs:element name="level">
  <xs:simpleType>
    <xs:restriction base="xs:string">
      <xs:enumeration value="beginner"/>
      <xs:enumeration value="intermediate"/>
      <xs:enumeration value="advanced"/>
    </xs:restriction>
  </xs:simpleType>
</xs:element>

A length limit

<xs:element name="username">
  <xs:simpleType>
    <xs:restriction base="xs:string">
      <xs:minLength value="4"/>
      <xs:maxLength value="20"/>
    </xs:restriction>
  </xs:simpleType>
</xs:element>

A pattern

A pattern uses a regular expression. The value must match the whole pattern. This one accepts a code such as AB-1234: two capital letters, a hyphen, then four digits.

<xs:element name="code">
  <xs:simpleType>
    <xs:restriction base="xs:string">
      <xs:pattern value="[A-Z]{2}-[0-9]{4}"/>
    </xs:restriction>
  </xs:simpleType>
</xs:element>

Naming your own types

If you use the same restriction in several places, give it a name and reuse it.

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <xs:simpleType name="percentType">
    <xs:restriction base="xs:integer">
      <xs:minInclusive value="0"/>
      <xs:maxInclusive value="100"/>
    </xs:restriction>
  </xs:simpleType>

  <xs:element name="result">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="maths" type="percentType"/>
        <xs:element name="science" type="percentType"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

</xs:schema>

Both maths and science must now be whole numbers from 0 to 100. Change the rule in one place, and it changes everywhere.

Lists and unions

• An xs:list type accepts several values separated by spaces, such as 1 2 3.
• An xs:union accepts a value that matches any one of several types.

They are less common, so look them up in the XML Schema reference when you need them.

A note on checking numbers

Types only describe format. A schema cannot check rules that depend on other values, such as “the end date must be after the start date”. Such rules need extra code, or newer schema features. Always keep your own validation in your program too.

Common mistakes

• Using xs:integer and expecting to forbid zero. Use xs:positiveInteger.
• Writing dates as 15/03/2026 instead of 2026-03-15.
• Forgetting that a pattern must match the whole value.
• Putting facets in the wrong place, outside the restriction.
• Using enumeration with different capital letters from the document. Values are case sensitive.
• Expecting a type to reject a value that is correct in format but wrong in meaning.

<xs:element name="grade">
  <xs:simpleType>
    <xs:restriction base="xs:string">
      <xs:enumeration value="A"/>
      <xs:enumeration value="B"/>
      <xs:enumeration value="C"/>
      <xs:enumeration value="D"/>
      <xs:enumeration value="F"/>
    </xs:restriction>
  </xs:simpleType>
</xs:element>

Summary

• XSD has built-in types for strings, numbers, booleans, dates, and more.
• Restrict a type with facets such as ranges, lengths, enumeration, and pattern.
• Name reusable types, and remember that types check format, not meaning.