why lexical variables can not be interpolated?

[lightoverhead]

Dear Monks

When I read perl cook book,I found the following code:

our ($rows, $cols);         #if use my ($rows,$cols), it will not work
no strict 'refs';                   # for ${$1}/g below
my $text;

($rows, $cols) = (24, 80);
$text = q(I am $rows high and $cols long);  # like single quotes!
$text =~ s/\$(\w+)/${$1}/g;
print $text;
#I am 24 high and 80 long

I am wondering why lexical variable (using my) will not work in this case.

Thank you!

Update:

Got the answer, sorry for this silly question.

symbolic reference is interpreted as a string representing the name of a global variable. So if I use symbolic reference as above, it has to refer to a global variable.

By the way, does anyone know if there will be a new version of Perl Cookbook?

Update questions:

Thank you for replying to my questions.

After playing around with this '/e' modifier with s///, I do have a question which I cannot solve now:

$AGE = 17;
$text = 'I am $AGE years old';      # note single quotes
$text =~ s/(\$\w+)/$1/eeg;           # print I am 17 years old
$text =~ s/(\$\w+)/$1 * 2/eegx;  #I tried to double the age, but faile
+d

How I can double the age in s/// format for the above code as showed by a similar but different example:

$text = "I am 17 years old";
$text =~ s/(\d+)/2 * $1/eg; #now I am 34 years old

Thank you.

[davido]

Regarding your "By the way" question... the most accurate answer will come from the OReilly website itself. Currently this is the most recent: Perl Cookbook, 2nd Edition, from 2003.

[kennethk]

Obligatory documentation reference: Symbolic references in perlref.

[lightoverhead]

Thank you for replying to my questions.

After playing around with this '/e' modifier with s///, I do have a question which I cannot solve now:

$AGE = 17;
$text = 'I am $AGE years old';      # note single quotes
$text =~ s/(\$\w+)/$1/eeg;           # print I am 17 years old
$text =~ s/(\$\w+)/$1 * 2/eegx;  #I tried to double the age, but faile
+d

How I can double the age in s/// format for the above code as showed by a similar but different example:

$text = "I am 17 years old";
$text =~ s/(\d+)/2 * $1/eg; #now I am 34 years old

Thank you.

[kennethk]

As described in s/PATTERN/REPLACEMENT/msixpodualgcer in perlop,

e Evaluate the right side as an expression.
ee Evaluate the right side as a string then eval the result.

The reason $text =~ s/(\$\w+)/$1/eeg; works is the regular expression stores $AGE (string literal) in $1. For the substitution, $1 is evaluated to return $AGE, and then $AGE is evaluated to return 17. Your second code attempts to multiply the string literal $AGE by 2, not the value of it - you've got your order of operations off. You could accomplish this using an explicit dereference rather than invoking eval twice, like

$text =~ s/\$(\w+)/$$1 * 2/egx;  #I tried to double the age and succee
+ded!

What I've changed:
1. I used an explicit dereference $$1, which could also be expressed as ${$1}
2. I changed the pattern so that the sigil is replaced, but is not part of the pattern. This is necessary because $$1 attempts to access the scalar variable named AGE; otherwise it would assume you are looking for a variable with an explicit dollar sign in its name.
3. I changed the code to eval only once.

I realize that this is a learning exercise, but for reference if I were going to do this kind of templating, I would use sprintf in something like:

$AGE = 17;
$tmpl = 'I am %s years old';      # note single quotes
$text = sprintf $tmpl, $AGE * 2;  # print I am 17 years old