Denis Roegel followed up,
| Actually, when I used what was suggested in earlier posts,
| namely to match FOUND against `.', it failed. I mean, somehow
| even when FOUND was empty, FOUND ?? . was true. I solved this
| by matching FOUND against `..'. For me, this is not a problem
| because I will always ensure that in the final rules something
| significant will be more than a character long.
Very probably the variable was not empty but contained one space or one tab.
I know from much personal experience that
* variable ?? .
fails if the variable is unset or null.
That was why I suggested
* FOUND ?? [^ ]
to require that $FOUND contain at least one character that is not a space, a
tab, nor a newline (nor a putative newline).