use strict; use warnings; use 5.010; my $str =<<"ENDOFSTRING"; Defaults env_reset Defaults requiretty ENDOFSTRING open(my $INPUT, '<', \$str) or die "Couldn't open for string io: $!"; while (my $line = <$INPUT>) { $line =~ s{ \s* ( Defaults \s* (env_reset | requiretty) ) \s* } {#$1}xms; say "-->$line<--"; } close $INPUT; --output:-- -->#Defaults env_reset<-- -->#Defaults requiretty<--