How can I quickly toggle between string and regexp versions of functions such as
query-replace-regexp etc? Pretty sure I saw a shortcut once but now I cant find it.
Like Harald, I'm not sure what kind of answer you expect.
But FWIW, I think the best way to perform such operations is with:
C-s <string1> M-% <string2> RET
in which case the answer is that you can use
M-r any time between the
C-s and the
M-% to toggle between string search and regexp search.
You can similarly use
M-c to toggle the case-sensitivity.