$_ = '  onn (bbcreccsnnl_output) !OUTPUT';
if ($_ =~ /^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";
  }

##</code><code>##

It matches with case insensitive...
It does NOT match without case insensitive...

##</code><code>##

($_ =~ /\(\s*IOPUT|OUTPUT\s*\)/)