Question

I want to be able to take an argument from the command line and use it as a regular expression within my script to filter lines from my file. A simple example

$ perl script.pl id_4

In script.pl:

...
my $exp = shift;
while(my $line = <$fh){
    if($line =~ /$exp/){
        print $line,"\n";
    }
}
...

My actual script is a bit more complicated and does other manipulations to the line to extract information and produce a different output. My problem is that I have situations where I want to filter out every line that contains “id_4” instead of only select lines containing “id_4”. Normally this could be achieved by

if($line !~ /$exp/)

but, if possible, I don’t want to alter my script to accept a more complex set of arguments (e.g. use !~ if second parameter is “ne”, and =~ if not).

Can anyone think of a regex that I can use (beside a long “id_1|id_2|id_3|id_5…”) to filter out lines containing one particular value out of many possibilities? I fear I’m asking for the daft here, and should probably just stick to the sensible and accept a further argument :/.

Answer 1

Why choose? Have both.

my $exp = join "|", grep !/^!/, @ARGV;
my @not = grep /^!/,  @ARGV;
s/^!// for @not;
my $exp_not = join "|", @not;

...
if (( $line =~ $exp ) && ( $line !~ $exp_not )) {
    # do stuff
}

Usage:

perl script.pl orange soda !light !diet