Remix can be any word (and would most likely be accompanied by another word) and in fact, it could be be any words in the brackets [I just chose Remix as I thought I could adapt the solution accordingly]. But its good that this is clarified.
Filename and Tag field please (am I correct in thinking if the expression is _ALL then this will replace in any filenames and tags where the character is found and for others it will just ignore it?).
And yes, there will be other text in the filename an example being: Title - Artist (ABC Remix(.mp3
Thanks for your help Florian, it is much appreciated.