[philsnow]

Seems to depend on your particular less, see the last sentence of this paragraph from `man less` on my machine:

       &pattern
              Display only lines which match the pattern; lines which do not
              match the pattern are not displayed.  If pattern is empty (if
              you type & immediately followed by ENTER), any filtering is
              turned off, and all lines are displayed.  While filtering is in
              effect, an ampersand is displayed at the beginning of the
              prompt, as a reminder that some lines in the file may be
              hidden.  Multiple & commands may be entered, in which case only
              lines which match all of the patterns will be displayed.

Indeed, I thought it would behave like you describe.. when I was refreshing my memory of how the negative pattern filtering worked, I first did &/pattern and then &/^Npattern, and was surprised to see that it displayed zero matching lines.

this is my version of less:

  % less --version
  less 581.2 (POSIX regular expressions)
  Copyright (C) 1984-2021  Mark Nudelman

[hddqsb]

Oh wow, that's a pretty new feature! It was added in v569, 2020[1].

[1]: https://github.com/gwsw/less/commit/6a070fc53799fb86e0fe3880...