Dear Monks,
Why does this match the regexp when using case insensitive matching? IMHO this should not match in either case and yet it does when I add case insensitivity. Here is the code: 
$_ = '  onn (bbcreccsnnl_output) !OUTPUT';
if ($_ =~ /^S\s+[-]*\d+[\.\d+]*\s+[-]*\d+[\.\d+]*\s*\(\s*IOPUT|OUTPUT\
+s*\)/i)
  {
    print "It matches with case insensitive...\n";
  }
else
  {
    print "It does NOT match with case insensitive...\n";
  }
if ($_ =~ /^S\s+[-]*\d+[\.\d+]*\s+[-]*\d+[\.\d+]*\s*\(\s*IOPUT|OUTPUT\
+s*\)/)
  {
    print "It matches without case insensitive...\n";
  }
else
  {
    print "It does NOT match without case insensitive...\n";
  }

[download]
And here is the output (on a linux box): 
It matches with case insensitive...
It does NOT match without case insensitive...

[download]
Your insights are greatly appreciated. This has me baffled (not that this is difficult to achieve or anything).

Update: it seems to have the same problem even with the much simpler regexp: 
($_ =~ /\(\s*IOPUT|OUTPUT\s*\)/)

[download]

In reply to Incorrect Pattern Matching Behavior by T.G. Cornholio

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