Strangely enough, you've hit a regular expression gotcha.

The empty regular expression has a special meaning: try again the last regular expression that successfully matched. What is perhaps less obvious is that perl checks for an empty expression after interpolating variables, such as the \Q$exp_sender\E in your example.

Here's a snippet to illustrate: 

  "test" =~ /t/;  # successful match
  my $var = "";
  print "matched 't'\n" if "t" =~ /\Q$var\E/;
  print "matched 'u'\n" if "u" =~ /\Q$var\E/;

[download]

A couple of alternative approaches, depending on what you're trying to achieve: 

  print "'$signer' is not '$exp_sender'\n"
      unless $signer eq $exp_sender;
  print "'$signer' is not '$exp_sender'\n"
      unless index($signer, $exp_sender) >= 0;
  print "'$signer' is not '$exp_sender'\n"
      unless $exp_sender eq '' || $signer eq $exp_sender;

[download]

Hugo

In reply to Re: Regular Expression gotchas ? by hv
in thread Regular Expression gotchas ? by the_Don

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