XML Schema Restrictions

Built-in data types like xs:string and xs:integer are broad. A string accepts any text — but what if you only want a 6-digit number, or a color that must be "red," "green," or "blue"? Restrictions in XML Schema let you narrow a base type down to exactly the range or pattern your data requires.

Think of restrictions like a visa application form. The form has a field for age — but the validator only accepts values between 18 and 65. The base type is "number," but the restriction narrows it to a specific range.

xs:restriction Syntax

<xs:simpleType name="TypeName">
  <xs:restriction base="xs:baseType">
    <!-- constraining facets go here -->
  </xs:restriction>
</xs:simpleType>

Constraining Facets

Facets are the rules inside an xs:restriction that limit the allowed values. Different base types support different facets.

1. Enumeration — List of Allowed Values

Enumeration restricts a value to a fixed set of options.

<xs:simpleType name="TrafficLightType">
  <xs:restriction base="xs:string">
    <xs:enumeration value="red"/>
    <xs:enumeration value="yellow"/>
    <xs:enumeration value="green"/>
  </xs:restriction>
</xs:simpleType>

Valid

<signal>green</signal>

Invalid

<signal>blue</signal>

2. minInclusive and maxInclusive — Range with Boundaries Included

<xs:simpleType name="AgeType">
  <xs:restriction base="xs:integer">
    <xs:minInclusive value="18"/>
    <xs:maxInclusive value="65"/>
  </xs:restriction>
</xs:simpleType>

Both 18 and 65 are valid. Values below 18 or above 65 fail validation.

3. minExclusive and maxExclusive — Range with Boundaries Excluded

<xs:simpleType name="PositiveDecimalType">
  <xs:restriction base="xs:decimal">
    <xs:minExclusive value="0"/>
  </xs:restriction>
</xs:simpleType>

Zero is not valid. Any number greater than zero passes. This is useful for prices — a price of 0 is not allowed.

4. minLength and maxLength — String Length Limits

<xs:simpleType name="PasswordType">
  <xs:restriction base="xs:string">
    <xs:minLength value="8"/>
    <xs:maxLength value="20"/>
  </xs:restriction>
</xs:simpleType>

Passwords must be between 8 and 20 characters. "abc" is too short. A 25-character string is too long.

5. length — Exact Length

<xs:simpleType name="IndianPINCode">
  <xs:restriction base="xs:string">
    <xs:length value="6"/>
  </xs:restriction>
</xs:simpleType>

The postal code must be exactly 6 characters. "110001" is valid. "11000" is invalid.

6. pattern — Regular Expression Format

Pattern uses a regular expression to define the allowed format. This is the most flexible facet.

Phone Number Pattern

<xs:simpleType name="IndianPhoneType">
  <xs:restriction base="xs:string">
    <xs:pattern value="\+91-[0-9]{10}"/>
  </xs:restriction>
</xs:simpleType>

Valid: +91-9876543210    Invalid: 9876543210 (missing +91-) or +91-98765 (too short)

Email Pattern (simplified)

<xs:simpleType name="EmailType">
  <xs:restriction base="xs:string">
    <xs:pattern value="[a-zA-Z0-9._%+\-]+@[a-zA-Z0-9.\-]+\.[a-zA-Z]{2,}"/>
  </xs:restriction>
</xs:simpleType>

7. totalDigits and fractionDigits — Decimal Precision

<xs:simpleType name="PriceType">
  <xs:restriction base="xs:decimal">
    <xs:totalDigits value="8"/>
    <xs:fractionDigits value="2"/>
  </xs:restriction>
</xs:simpleType>

This allows up to 8 total digits with exactly 2 decimal places. Valid: 12345.67    Invalid: 12345.678 (3 decimal places)

8. whiteSpace — Whitespace Handling

<xs:simpleType name="CleanStringType">
  <xs:restriction base="xs:string">
    <xs:whiteSpace value="collapse"/>
  </xs:restriction>
</xs:simpleType>

Combining Multiple Facets

Multiple facets apply together — all conditions must be satisfied.

<xs:simpleType name="ProductCodeType">
  <xs:restriction base="xs:string">
    <xs:length value="7"/>
    <xs:pattern value="[A-Z]{2}-[0-9]{4}"/>
  </xs:restriction>
</xs:simpleType>

A product code must be exactly 7 characters and match the format: two uppercase letters, a hyphen, four digits. Valid: PC-0042    Invalid: pc-0042 (lowercase) or P-0042 (wrong length)

Using Custom Types in Element and Attribute Declarations

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

  <xs:simpleType name="AgeType">
    <xs:restriction base="xs:integer">
      <xs:minInclusive value="0"/>
      <xs:maxInclusive value="120"/>
    </xs:restriction>
  </xs:simpleType>

  <xs:simpleType name="IndianPINType">
    <xs:restriction base="xs:string">
      <xs:pattern value="[1-9][0-9]{5}"/>
    </xs:restriction>
  </xs:simpleType>

  <xs:element name="person">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="name"    type="xs:string"/>
        <xs:element name="age"     type="AgeType"/>
        <xs:element name="pinCode" type="IndianPINType"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>

</xs:schema>

Facet Quick Reference

Key Points to Remember

• xs:restriction narrows a base type using constraining facets.
• Enumeration limits values to a specific list.
• minInclusive and maxInclusive set a range where both boundary values are valid.
• minExclusive and maxExclusive set a range where boundary values are not valid.
• pattern uses regular expressions to enforce a specific text format.
• length, minLength, and maxLength control how long a string can be.
• Multiple facets combine — the value must satisfy all of them simultaneously.