Regular expression with (?i:...) non-capturing group in @Path annotation

I wanted to make case/insensitive API, so I had to use regular exception in @Path annotations. This works:

@Path("/

{api:[aA][pP][iI]}

")
public class Api {
@Path("/

{func:[fF][uU][nN][cC]}

")
public void func()

{ ... }
}

I tried to change weird regular exception to clearer one, based on non-capturing group (?:...).

@Path("/{api:(?i:api)}")
public class Api {
@Path("/{func:(?i:func)}")
public void func() { ... }

}

but exception "java.lang.IndexOutOfBoundsException: No group 3" is thrown on any request.
When I use (i?:...) only on class Api level and not on method func(), it surprisingly works.