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
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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
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Thank you.

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+)/$$1 * 2/egx;  #I tried to double the age and succee
+ded!
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What I've changed:
1. I used an explicit dereference $$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
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#11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.

kennethk

Thank you for your detailed answers to this question.

I know if I use a symbolic reference will get this done. But it will not work for lexical scope variable. If, for example, I use:

my $AGE =17 #instead of $AGE=17
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your solution will fail.

It seems to me that it's impossible to use /e to solve this situation when variable is lexical scoped.

Yes, sprintf will work for this case as we know it's a number; but in other cases we may not know if $AGE holds a numeric value.

Any thoughts?

Thank you!