You can give it something else than defaultXPConfig to customize the look and completion behavior: http://xmonad.org/xmonad-docs/xmonad-contrib/XMonad-Prompt.h...
Setting:
searchPredicate :: String -> String -> Bool
searchPredicate = \_ _ -> False
And the default is:
searchPredicate = isPrefixOf
[1]: https://code.google.com/p/xmonad/issues/attachmentText?id=39... [2]: https://code.google.com/p/xmonad/issues/detail?id=393