How do I assign a shortcut to something if it isn't listed in User Preferences' shortcut list? I'm trying to assign a shortcut on the user preferences to the "select pattern" command, but it doesn't seem to exist.
The Input panel in the User Preferences is for searching for shortcuts that already exist. As there isn't a shortcut for Select Pattern already, it won't appear in there.

To add a shortcut, go to the menu where Select Pattern is (Object Mode> Select> Select Pattern...), right-click and choose Add Shortcut. After entering a shortcut it will now be present in the User Preferences when you search for it.