XML Schema Definition (XSD) validation is a useful mechanism to validate data in the XML parser, before it reaches application logic, for example analyzing web service input at a web services gateway.  There are drawbacks to validating data via XSD, foremost that it makes analyzing specific errors in the data difficult, however the easy maintainability of an XSD and the separation from application logic generally make up for the shortcomings.

XSD data validation is typically handled by the xs:restriction XSD element, though it is limited to xs:simpleType and xs:simpleContent declarations.  Below is an introduction to using the various restrictions of xs:restriction:

xs:minExclusive and xs:maxExclusive

minExclusive and maxExclusive define exclusive lower and upper bounds for an element, analogous to the > and < mathematical inequalities.  Setting minExclusive to 0 would mandate that only values greater than 0 are accepted, while a maxExclusive of 100 would mandate that only values less than 100 are accepted.

Example:

Restrictions on Use:

• xs:minExclusive may not be used simultaneously with xs:minInclusive.  Similarly xs:maxExclusive may not be used simultaneously with xs:maxInclusive.
• xs:minExclusive and xs:maxExclusive must be valid with respect to the base type specified in the parent xsd:restriction.  For example, if the base type is an integer, specifying a decimal value for either minExclusive or maxExclusive is invalid.
• xs:minExclusive must be a smaller value than any complementary xs:maxExclusive or xs:maxInclusive elements.  Similarly xs:maxExclusive must be a larger value than any complementary xs:minExclusive or xs:minInclusive elements.

xs:minInclusive and xs:maxInclusive

minInclusive and maxInclusive define inclusive lower and upper bounds for an element, analogous to the >= and <= mathematical inequalities.  Setting minInclusive to 0 would mandate that only values greater than or equal to 0 are accepted, while a maxInclusive of 100 would mandate that only values less than or equal to 100 are accepted.

Example:

Restrictions on Use:

• xs: minInclusive may not be used simultaneously with xs: minExclusive.  Similarly xs: maxInclusive may not be used simultaneously with xs: maxExclusive.
• xs: minInclusive and xs: maxInclusive must be valid with respect to the base type specified in the parent xsd:restriction.  For example, if the base type is an integer, specifying a decimal value for either minInclusive or maxInclusive is  invalid.
• xs: minInclusive must be a smaller value than any complementary xs:maxExclusive or xs:maxInclusive elements.  Similarly xs: maxInclusive must be a larger value than any complementary xs:minExclusive or xs:minInclusive elements.

xs:totalDigits

totalDigits restricts the maximum number of digits allowed for a numeric data element however fewer digits may be provided and pass validation.

Example:

xs:fractionDigits

fractionDigits indicates the maximum number of digits in the fractional part (the part to the right of the decimal) of a numeric data element derived from decimal base type.

Example:

xs:length

length specifies the exact length of an element, either in number of characters for string data or number of octets for binary data (type hexBinary or base64Binary).  Provided data may not be shorter or longer; it may only be exactly the specified length to pass validation.

Example:

Restrictions on Use:

• xs: length may not be used simultaneously with xs: minLength or xs:maxLength.
• xs:length is a non-negative integer

xs:minLength and xs:maxLength

minLength and maxLength define inclusive lower and upper lengths for an element, analogous to the >= and <= mathematical inequalities.  minLength and maxLength correspond to either the number of characters desired for string data,  number of octets desired for binary data (type hexBinary or base64Binary), or number of items desired for List Items.

Example:

Restrictions on Use:

• xs: minLength and xs:maxLength may not be used simultaneously with xs: length.
• xs:minlength and xs:maxLength are non-negative integers, and minLength is a value less than or equal to maxLength if both are used

xs:enumeration

enumeration defines an enumeration of values for a data element.  Any values not within the enumeration cause validation to fail.  This is useful for validating data elements for a fixed and small set of accepted values.

Example:

xs:whitespace

Rather than acting specifically as a validation directive, whitespace specifies how the XML validator should normalize whitespace content prior to handing the data off to the application.  Xs:whitespace may be one of the following three values:

• preserve: No normalization takes place, the value remains unchanged.
• replace: Each occurrence of the ASCII characters #x9 (tab), #xA (line feed) and #xD (carriage return) is substituted by #x20 (space).
• collapse: The same processing as replace, but additionally each sequence of one or more consecutive #x20 (space) characters is converted to a single #x20

xs:pattern

Pattern validates a string data element using a specified regular expression.  Multiple xs:pattern elements can be included within a given xs:restriction element; if done the validator will accept any data that passes is matched by at least one of the included patterns.  A list of common regular expressions can be found here.

Example: