I've got a file that I want to convert to a CSV format.

Each line of the file is something like:

term1a term1b [pronunciation] definition
term2a term2b [pronunciation] definition
term3a term3b [pronunciation] definition

To get it into CSV, I just need to perform 3 find and replace operations:

First, replace the first " " (space) with a "," (comma).
Next, replace the first " [" (space and right-facing square bracket) with a "," (comma).
Finally, replace the first "] " (left-facing square bracket and a space) with a "," (comma).

The reason it must only be the first such occurance, is the definition area contains lots of spaces and square brackets which I'd like to leave left untouched.

Ok...I'd definitely use 'sed' to do that. You should easily be able to write a script to do it.