This highlights the parts of the lines that are over 80 columns long:
augroup over_length autocmd! autocmd BufEnter * highlight OverLength ctermbg=darkgrey autocmd BufEnter * match OverLength /\%80v.*/ augroup END
However, I would like to modify it to use
/\%.&textwidth.v.*/ doesn't seem to work. Is there a way to substitute the value into the regular expression?