Using java regular expressions to match groups

It's been demonstrated over and over again, but I always seem to forget the "simple example". So here it is:

import java.util.regex.Matcher;
import java.util.regex.Pattern;
...
/*
 *Going to match a URL that looks like this 
 * /url/prefix/--sitename--/menu.--menuid--/?param=--itemid--
 * and change it into 
 * /url/prefix/--sitename--/--menuname--/--itemname--
*/
String expr="(/url/prefix/[a-zA-Z0-9_-]+)/menu\\.([a-zA-Z0-9_-]+)/\\?param=([a-zA-Z0-9_-]+)";
Pattern p = Pattern.compile(expr);
Matcher m = p.matcher(uri);
if ( m.matches() ) {
    prefix= m.group(1);
    menuitem = m.group(2);
    guid = m.group(3);
    if ( guid != null && prefix != null && menuitem != null ) {
        uribuilder.append(prefix);
        uribuilder.append("/");
        uribuilder.append(getMenuName(menuitem));
        uribuilder.append("/");
        uribuilder.append(getItemName(guid));
        uri=uribuilder.toString();
    }
}