I needed a function with the following properties: prefix_print(@list) should work exactly like print(@list), except that each line in the output should be prefixed by $prefix. For example, if $prefix is set to 'P', a call to prefix_print("A\nB","C\n") should print: 

PA
PBC

[download]

My first attempt went like this: 


sub prefix_print {
  # Put $prefix in front
  my $outstr=join('',$prefix,@_);
  # If something follows a \n, insert $prefix
  $outstr =~ s/(\n)(?=.)/$1.$prefix/egs;
  print $outstr;
}

[download]
This code works, as far I can see, but it requires the usage of a temporary variable, $outstr. Of course this is not really a problem, but being very fond of Perl's functional aspects (for instance, using split, map and so on), and - more out of curiosity than necessity - I was searching for an alternative solution, working without temporary variable and still simple enought to be understandable. Here is my attempt: 

# Note: This is buggy
sub prefix_print {
  # Turn arguments into a list of lines, and prepend
  # each with $prefix
  print(
    map { "$prefix$_" } 
        join('',@_) =~ m/(.*(?:\n|$))/g
  );
}

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This does NOT work, however, because if called as prefix_print('y'), it prints 

PyPP

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which suggests that my regular expression returns an array of three matches (one containing 'y', and two containing the empty string).

Could someone explain where these matches come from? Also, I would be glad to learn about any improvements for my code.

-- 
Ronald Fischer <ynnor@mm.st>

In reply to Can we do without auxiliary variable here? by rovf

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