# % expansion.  %% gets converted to % later, so expand any
# %keyword construction that doesn't have a % in front of it
# (modulo multiple %% pairs in between).
while (($str =~ s/(^|[^\%](?:\%\%)*)\%([_a-zA-Z]\w*)/"$1".$env->{$2}/ge) ||
       ($str =~ s/(^|[^\%](?:\%\%)*)\%\{([_a-zA-Z]\w*)\}/"$1".$env->{$2}/ge)) {}