A matching rule defines a specific way to compare attribute values that have a given syntax. In other words, a matching rule defines how potentially matching attributes are compared.
Every matching rule is identified by a unique object identifier (OID) string. A client application that requests a search can specify the matching rule OID in the search filter. The OID indicates to Directory Server how to check for a match of two attribute values.
In practice, a client application might want only entries with attribute values that match the value provided exactly. The sample plug-in demonstrates how you might implement a solution for that case. Another client might want to sort entries according to the rules for a given locale. Directory Server actually uses a matching rule plug-in to handle locale—specific matching.
Chapter 11, Directory Server Internationalization Support, in Sun Java System Directory Server Enterprise Edition 6.0 Reference includes a list of matching rules. Directory Server supports the rules for internationalized searches. You can also view the list by searching the default schema.
$ ldapsearch -h localhost -p 1389 -b cn=schema cn=schema matchingRules