Patterns for Identifiers

Identifiers are all around us. Social Security numbers, zip codes, and DUNS numbers (unique numbers issued by Dun & Bradstreet and used to identify business entities) are just a few. The range of values of such things is usually way too large and dynamic to practically enumerate in a schema, although business applications usually validate them in their databases. Even though we might not want to enumerate the set of allowable values, we can do a level of validation by specifying a pattern to which the identifier must conform. For example, U.S. Social Security numbers have the general pattern NNN-NN-NNNN, where N represents a digit from 0 through 9. U.S. zip codes (in the five-digit form) have a pattern of NNNNN. The common way to declare an identifier type is to restrict the string data type using the pattern facet. Here's an example for zip codes.

Specifying a Pattern in SimpleCSV2.xsd

<xs:simpleType name="zipCodeType">
  <xs:annotation>
    <xs:documentation>Here we define a ZIP Code as 5-digit
        pattern.
    </xs:documentation>
  </xs:annotation>
  <xs:restriction base="xs:string">
    <xs:pattern value="ddddd"/>
  </xs:restriction>
</xs:simpleType>

Each d in the value Attribute of the xs:pattern Element indicates a decimal digit. Schema language offers many ways to specify patterns. Patterns are specified as regular expressions, which are sequences of characters that denote sets of strings. The schema language notation for regular expressions is very similar to that used in Perl. It is described in Appendix F, Part 2, of the Schema Recommendation. However, that appendix is somewhat obtuse and doesn't have any examples. If you're going to use patterns frequently and are new to regular expressions, I recommend getting a good introductory book on Perl, learning how Perl does regular expressions, then seeing how schema language differs. There's also a good resource on the Web, listed at the end of this chapter.