It's hard to tell from your example exactly what the pattern is that you're looking to match, but since you said that's not the difficult part, I'll just assume you're more interested in the general principle and not the specific regex. So here's my approach, which works with the example lines you provided:
nnoremap <Leader>f :execute 'edit' substitute(getline('.'),
'\v.*\w{3}\.(\w+)\.\w{3}', '\1', '') . '.txt'<CR>
Of course, you can use any key mapping, I just arbitrarily picked
<Leader>f for the sake of example.
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