Demonstrate usage of expressions in Replace text.
This particular command converted -
--
--autocmd BufEnter *.c nmap <F5> :Makexec<CR> :if findfile( expand("%:p:r").".exe" ,expand("%:p:h") )!=""<CR> :!%:p:r.exe<CR> :else<CR> :vert topleft cwin<CR> :vert resize 50<CR> :endif<CR>
--

to

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--autocmd BufEnter *.c nmap <F5> :Makexec<CR>
--                              \:if findfile( expand("%:p:r").".exe" ,expand("%:p:h") )!=""<CR>
--                              \    :!%:p:r.exe<CR>
--                              \:else<CR>
--                              \    :vert topleft cwin<CR>
--                              \    :vert resize 50<CR>
--                              \:endif<CR>
--

NOTE: The usage of \= represents beginning of an expression only when placed at the start of the replace text; elsewhere it has a different meaning.