Using perl, how do you process a regex taken from command line input?

Question

I'm creating a script to take regex input from the command line and process it; something like this:

chomp(my $regex = $ARGV[0]);
my $name = '11528734-3.jpg';

$name =~ $regex;

print $name . "\n";

My input into the script is: "s/.jpg/_thumbnail.jpg/g" but $name isn't processing the regex input from the command line.

Any advice on how to make this work?

Thanks!

You are doing a regex match but not checking whether it succeeded or using its results in any way. What change to you want to happen to $name? — ysth
– ysth, Commented Aug 7, 2011 at 5:41
@ysth: I have a feeling that the OP had the argument in the form of s/foo/bar/ and was expecting the =~ $regex to "expand" to =~ s/foo/bar/. — C. K. Young
– C. K. Young, Commented Aug 7, 2011 at 5:47
Now that you have added what your input is, your title/question is wrong. You are not trying to take a regex from the command line, you are trying to take an operator from the command line. — tadmc
– tadmc, Commented Aug 7, 2011 at 20:23

C. K. Young · Accepted Answer · 2011-08-07 05:44:28Z

9

Using $name =~ $regex won't change your $name. You have to use the s/// operator to effect any change.

e.g.,

$name =~ s/$pattern/$replacement/;

If you are specifying both the pattern and replacement in the same argument, e.g., in the form of s/foo/bar/, you will have to split them first:

my (undef, $pattern, $replacement) = split '/', $regex;
$name =~ s/$pattern/$replacement/;

Original answer:

Use qr//:

$name =~ qr/$regex/;

You can also just use $name =~ /$regex/, but the qr version is more general, in that you can store the regex object for later use:

$compiled = qr/$regex/;
$name =~ $compiled;
$name =~ s/$compiled/foobar/;

etc.

edited Aug 7, 2011 at 5:44

answered Aug 7, 2011 at 4:23

C. K. Young

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3 Comments

ysth Over a year ago

shouldn't make any difference; just =~ $regex should work the same

C. K. Young Over a year ago

@ysth: In that case, the OP's problem is that he's not using s///, and thus the string isn't modified. :-)

Francis Lewis Over a year ago

Splitting the string works, but is there any way ... besides maybe using eval ... to pull in whatever regex is sent into the script? I updated the original question with my input into the script as well.

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