Coded up an assignment in C99 but the uni server is only C89. One of the main issues was declaring counters in the for-loop. This the regexp I used in VIM to move counteres outside.

c99-to-c89-for-loop-regexp.vim

" This was a massive help for me cause I could just run one command on a file and it would compile to C89. 
" Hopefully this is useful to anyone who runs into the same problem :)
" It's a bit long winded so I put an explanation below:
" 1 = spacing, this allows the counter declaration to be easily indented the same amount no matter the 
"      indentation.
" 2 = 'for ('
" 3 = the counter type
" 4 = is for if the counter is pointer, like if you're iterating over a linked list
" 5 = the counter variable name
" 6 = the rest of the for-loop

%s/\(\s\+\)\(for (\)\(\w\+\) \(*\?\)\(\w\+\)\( .\+\)/\1\3 \4\5;\r\1\2\5\6/ 

" \s = whitespace
" \+ = one or more
" \? = one or none
" . = anything but an end of line character
" \<n> = refers to the nth grouping between \( and \)

Relevant xkcd