Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Dear perl monks: The following code (or something like it) will check the RE in $re for proper syntax. However, -w finds other useful trouble conditions, which it reports to STDERR. I would like to detect these warnings and act on them, and not display them. Can this be done? 
#!/usr/bin/perl -w

$re = '[\w-]';
eval {'' =~ /$re/ }
if($@) {print "error: $@\n"}

[download]

Replies are listed 'Best First'.
Re: trapping -w warnings
by Zaxo (Archbishop) on Nov 26, 2003 at 07:08 UTC

    You can set up a $SIG{'__WARN__'} handler. Doing that suppresses printing the message to STDERR. See the perlvar article on %SIG for details.

    After Compline,
    Zaxo

[reply]
      What Zaxo doesn't provide, is a working example. So, here is one, tailored to your snippet: 
      $re = '[\w-]';
my @warnings;
{
    local $SIG{__WARN__} = sub { push @warnings, shift };
    eval {'' =~ /$re/ };
}
if($@) {
    print "error: $@\n";
} else {
    foreach my $warning (@warnings) {
        print "Got a warning: $warning";
    }
}

      [download]
      Alternatively, you can do your checking inside the $SIG{__WARN__} sub.

      Note how you need a semicolon after some "blocks" for it to work: after the assignment of the anonymous sub to $SIG{__WARN__}, and after the eval block, at least if any statement comes right after it — which you forgot.

[reply]
[d/l]
        Perfect. Thanks to both of you. I gather that the effect of "local" as used here is to restore the original handler upon exiting the block?
[reply]
        I liked this enough that I bookmarked this in my delicious collection -- helped me debug a nasty problem.

        I did things slightly differently though -- more localized, I think. 

            {
      local $SIG{__WARN__} = sub {
        my @warnings = @_;
        foreach my $warning (@warnings) {
            print "got a warning: $warning\n";
          }
      };
      eval {'' =~ /$re/ };
    }

        [download]
        cheers.
[reply]
[d/l]

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