The following keys can be used to initiate a search:
/ and Ctrl+F.
F3, A, Ctrl+L, Ctrl+N
Ctrl+F3 can be used to search for the next occurrence of any highlighted text.
Alt+A (and Alt+F3) will continue searching the "next file" if no further matches are found in the file being viewed.
To search backwards, use any of the Find Next keys with the SHIFT key pressed.
Using ? will always initiate a backwards search. That is, a normal search with the Direction option set to Back.
Match Case toggle
Using \ will initiate a search with the default "Match Case" option toggled.
That is, if you perform a normal search (eg, using /) with the Match Case option disabled, pressing "\" will initiate a search with the Match Case option disabled. If you perform a normal search with the Match Case option enabled, using "\" will disable the Match Case option.
You can disable this behavior by enabling Disable "Match Case Toggling" (using the \ key).