Doubt

[anand kumar]

  Hi 
         I am new to perl .i have a doubt in analysing the following regex.
   (my $book = $ref_string) =~ s/\s*(\d+(?::\d+(?:-\d+(?::\d+)?)?)?)\Z//;
   
  here i want to know the meaning of '?:'
   
  thanks in advance
  Anand

                
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