$& as a read-only value

frasco has asked for the wisdom of the Perl Monks concerning the following question:

dear monks, I hope this question is not so stupid but I can solve small point. I would like to substitute some of the values of a string line. Thus I wrote the following snippet:

my @array = qw('a3-da-um-=TUG2-:II 1 AKTUM-=TUG2 1 IB2-:4-=TU[G2 S[A]6
+ DAR);
   for my $word(@array) {
      if ($word =~ /(\(U)(\[|\])?(\w)?(2)/) {
         $& =~ s/$1/Ú/;
         $$ =~ s/$4//;
   }
print "@array\n";
[download]

well, I'm tring to substitute the matched value "UG2" with "ÚG", but when the script is loaded it return the following message: "Modification of a read-only value attempted at line ...". It refers to $&, but I don't understand what this message means.
Thank you

Comment on $& as a read-only value Download Code

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Re: $& as a read-only value by moritz (Cardinal) on May 22, 2008 at 17:26 UTC
It means that you can't modify `$&`. It's a read-only copy of the previous match. You can use a substitution instead: `$word =~ s/(whatever)U(whatever)/$1Ú$2/` [download] Notice that you can't modify `$$` either, which is the current process ID.	[reply] [d/l] [select]
Re: $& as a read-only value by Erez (Priest) on May 22, 2008 at 17:39 UTC
Apart from the solution suggestion above, a general warning, don't use $&, $`, or $'. The appearance of either of those variables in a program will grind its performance to a halt. Stop saying 'script'. Stop saying 'line-noise'. We have nothing to lose but our metaphors.	[reply]
Re^2: $& as a read-only value by moritz (Cardinal) on May 22, 2008 at 18:01 UTC
While I agree in general, I'd not call it grind its performance to a halt. If I remember correctly from previous explanations the impact of using one of those variables makes perl copy every string that you match a regex against. How much that is strongly depend on your application, and might range from "not measurable" to "makes your application crash" (when it runs out of memory). I find these variables very useful when debugging complicated regex. I put something like this into the regexes at various places: (?{ print "&`Ť$&ť$'\n" }) which shows pretty nicely how far the regex engine proceeded so far. (But you won't find that in production code ;-)	[reply] [d/l]
Re^3: $& as a read-only value by Erez (Priest) on May 22, 2008 at 18:49 UTC
I'd not call it grind its performance to a halt. The terminology might not be exact in terms of actual performance, but I found that phrase to leave an adequate impression every time the match variable and its kin appear in someone's code. You might say its fear mongering (and you might be right), but I'd rather exaggerate now, and debate facts after the usage of the match variables has been abolished (another strong word). Stop saying 'script'. Stop saying 'line-noise'. We have nothing to lose but our metaphors.	[reply]
Re^2: $& as a read-only value by almut (Canon) on May 22, 2008 at 18:04 UTC
The Devel::SawAmpersand module has a bit more info on why: There's a global variable in the perl source, called `PL_sawampersand`. It gets set to true in that moment in which the parser sees one of $`, `$'`, and `$&`. It never can be set to false again. Trying to set it to false breaks the handling of the $`, `$&`, and `$'` completely. If the global variable `PL_sawampersand` is set to true, all subsequent RE operations will be accompanied by massive in-memory copying, because there is nobody in the perl source who could predict, when the (necessary) copy for the ampersand family will be needed. So all subsequent REs are considerable slower than necessary.	[reply] [d/l] [select]