4.2. @PathParam and PathSegment

The specification has a very simple abstraction for examining a fragment of the URI path being invoked on javax.ws.rs.core.PathSegment: 

public interface PathSegment {

    /**
     * Get the path segment.
     * <p>
     * @return the path segment
     */
    String getPath();
    /**
     * Get a map of the matrix parameters associated with the path segment
     * @return the map of matrix parameters
     */
    MultivaluedMap<String, String> getMatrixParameters();
    
}
RESTEasy can inject a PathSegment instead of a value with your @PathParam. 
   @GET
   @Path("/book/{id}")
   public String getBook(@PathParam("id") PathSegment id) {...}
This is particularly useful when you have multiple @PathParams that use matrix parameters. Matrix parameters are an arbitrary set of name-value pairs embedded in a URI path segment. The PathSegment object fives you access to these parameters. See Chapter 7, @MatrixParam for further information.
An example of a matrix parameter: 
  GET http://host.com/library/book;name=EJB 3.0;author=Bill Burke
A matrix parameter represents a resource that can be addressed by its attributes as well as its raw ID.