>perl -wMstrict -le "sub rx (&@) { return map { local $_ = $_; ()= $_[0]->(); $_; } @_[1..$#_]; } ;; my $s = 'a b c d'; print rx { s/ /_/g } $s; ;; my @ra = ('p q r', 'x y z z y'); print rx { s/ /_/g } @ra; ;; my $x = q{Here are [[a [[ variable]] number [[of ]words]] in brackets}; print qq{'$x'}; ;; $x =~ s( \[\[ (.*?) \]\] )( (rx {s/ /_/g} $1)[0] )xmsge; print qq{'$x'}; " a_b_c_d p_q_rx_y_z_z_y 'Here are [[a [[ variable]] number [[of ]words]] in brackets' 'Here are a_[[_variable number of_]words in brackets'