$NetPattern = qr{mbist[\w+\d+\/]*\[\d+\]}x;
if ($NetName =~  $NetPattern) {# Do fantastical things...}
[download]

$NetPattern = 'mbist[\w+\d+\/]*\[\d+\]';
if ($NetName =~ qr/$NetPattern/x) {# Do fantastical things...}
[download]

The "" style quotes interpolated your slashes away.

The qr is also most useful (in my opinion) for precompiling regular expressions, so I would choose the first solution, particularly if you have a lot of matches to do on that single regular expression. It's also nice for being able to tell that $NetPattern is of type Regexp. So, I guess there's also this third choice...

$NetPattern = 'mbist[\w+\d+\/]*\[\d+\]';
if ($NetName =~ m/$NetPattern/x) {# Do fantastical things...}
[download]

Surely all these replies that fix one example out of an infinite number of possible user inputs are missing the point?

If I understood the original post, $NetPattern is to be read from the command line, so the example, mistaken bracketing and all, was just an example. The question was

If a user enters a command-line regular expression that gets pushed into a variable, what is the most reliable way to use that variable to compare it against another variable that is a string?

In our answers, we should be explaining how to validate a user-supplied string before interpolating it into a regex. A simple error like [ without matching ] is the least of our worries.

That's an excellent point actually. I'll amend my answer, although I covered it somewhat. Assuming you actually trust the user, this seems like a sane way to do it.

my $regexp = <$file_handle>;
my $creg   = eval { qr($regexp) };

if( $@ ) {
   warn "regex error: $@";

} else {
   if( $something =~ $creg ) {
      # do wonderful things.
   }
}
[download]

I apparently skipped the meat of the question because it seemed clear to me that the reason the matching failed was because of an interpolation problem with a test regexp that he generated between quotes and he wasn't actually reading it from the user -- which would have likely worked the way he had it.

-Paul

Unrecognized escape \w passed through at ./683940.pl line 6.
Unrecognized escape \d passed through at ./683940.pl line 6.
Unrecognized escape \d passed through at ./683940.pl line 6.
[download]

This is my version of your code:

#!/usr/bin/env perl
use warnings;
use strict;

my $NetName = 'top_level/mbist_wrapper_pix2d[22]';
my $NetPattern = "mbist[\w+\d+\/]*\[\d+\]";
if ($NetName =~ qr/$NetPattern/x) {
    print "match\n";
}
else{
    print "no match\n";
}
[download]

One solution is to avoid interpolation (as jettero has already mentioned).

#!/usr/bin/env perl
use warnings;
use strict;

my $NetName    = 'top_level/mbist_wrapper_pix2d[22]';
my $NetPattern = 'mbist[\w+\d+\/]*\[\d+\]';
if ($NetName =~ qr/$NetPattern/x) {
    print "match\n";
}
else{
    print "no match\n";
}
[download]

So...

perl -wMstrict -e
"my $NetPattern = 'mbist [\w/+]* \[ \d+ \]';
 my $NetName = 'top_level/mbist_wrapper_pix2d[22]';
 if ($NetName =~ qr/$NetPattern/x) {
  print 'match'
  }
"
match
[download]

Or perhaps better yet...

perl -wMstrict -e
"my $NetPattern = qr{mbist [\w/+]* \[ \d+ \]}xms;
 my $NetName = 'top_level/mbist_wrapper_pix2d[22]';
 if ($NetName =~ $NetPattern) {
 print 'match'
 }
"
match
[download]