Sorry, I didn't mean to imply that that M was at the beginning of the file. Each text file is a separate essay and the acronym was just a word in the body of the file.

I combined ideas from both tips to come up with the following (semi-Perl) solution. Thanks for your help Paladin and allolex.

for %a in (*.txt) do @perl -ne "if (m/ (M[A-Z]{3}) /) { print qq($1\n); }" %a