Xcode 5.1 method search only allows one character?
(This one's just informational: I don't think it will fit in a comment, and want to report on what suggestions worked and didn't.)
I ran into this same problem. I accidentally typed some other key combo when trying to do Ctrl+6 to open the Document Items dropdown, and it was all out of whack after that.
As clance_911 mentioned, the filtering would work after clicking in the search box. So for example, to filter for "init", I could hit Ctrl+6 to open the Document Items, type "i" to start filtering (but then any subsequent letters would replace the i), click into the search text box, and continue typing the "nit". This works, but it's a pain.
As Moze pointed out, this seems to be specific to external displays. Sure enough, it worked fine on my MacBook display, but if I moved Xcode over to the external display, it stopped working. In my case, dragging the menu bar in the display Arrangements settings (to make the external monitor the main display) did solve the problem: the filter worked correctly with Xcode in either window. Sorry, Etienne :-(
The other solution that worked for me was simply closing the MacBook: use it in clamshell mode with only the external display. This is my normal setup anyway, but I know that's not ideal for everyone.