Question

I am trying to write a simple perl script to apply a given regex to a filename among other things, and I am having trouble passing a regex into the script as an argument.

What I would like to be able to do is somthing like this:

> myscript 's/hi/bye/i' hi.h
bye.h
>

I have produced this code

#!/utils/bin/perl -w
use strict;
use warnings;

my $n_args = $#ARGV + 1;
my $regex =  $ARGV[0];
for(my $i=1; $i<$n_args; $i++) {
  my $file = $ARGV[$i];

  $file =~ $regex;
  print "OUTPUT: $file\n";
}

I cannot use qr because apparently it cannot be used on replacing regexes (although my source for this is a forum post so I’m happy to be proved wrong).

I would rather avoid passing the two parts in as seperate strings and manually doing the regex in the perl script.

Is it possible to pass the regex as an argument like this, and if so what is the best way to do it?

Answer 1

There’s more than one way to do it, I think.

The Evial Way:

As you basically send in a regex expression, it can be evaluated to get the result. Like this:

my @args = ('s/hi/bye/', 'hi.h');
my ($regex, @filenames) = @args;
for my $file (@filenames) {
  eval("\$file =~ $regex");
  print "OUTPUT: $file\n";
}

Of course, following this way will open you to some very nasty surprises. For example, consider passing this set of arguments:

...
my @args = ('s/hi/bye/; print qq{MINE IS AN EVIL LAUGH!\n}', 'hi.h');
...

Yes, it will laugh at you most evailly.

The Safe Way:

my ($regex_expr, @filenames) = @args;
my ($substr, $replace) = $regex_expr =~ m#^s/((?:[^/]|\\/)+)/((?:[^/]|\\/)+)/#;
for my $file (@filenames) {
  $file =~ s/$substr/$replace/;
  print "OUTPUT: $file\n";
}

As you can see, we parse the expression given to us into two parts, then use these parts to build a full operator. Obviously, this approach is less flexible, but, of course, it’s much more safe.

The Easiest Way:

my ($search, $replace, @filenames) = @args;
for my $file (@filenames) {
  $file =~ s/$search/$replace/;
  print "OUTPUT: $file\n";
}

Yes, that’s right – no regex parsing at all! What happens here is we decided to take two arguments – ‘search pattern’ and ‘replacement string’ – instead of a single one. Will it make our script less flexible than the previous one? No, as we still had to parse the regex expression more-or-less regularly. But now user clearly understand all the data that is given to a command, which is usually quite an improvement. )

@args in both examples corresponds to @ARGV array.