Perl Regex 'e' (eval) modifier with s///

Question

I'm having a little trouble comprehending this simple use of the /e regex modifier.

my $var = 'testing'; $_ = 'In this string we are $var the "e" modifier.';  s/(\$\w+)/$1/ee;  print; 

Returns: "In this string we are testing the "e" modifier."

I cannot see why two 'e' modifiers are required. As far as I can see, $1 should capture '$var' from the string and a single 'e' modifier should then be able to replace the variable with its value. I must be misunderstanding something however, since trying the above code with just one 'e' modifier does not visibly replace anything in the string.

Excuse me for asking such a simple question!

Thanks.

Answer 1

It’s not exactly a “simple” question, so don’t beat yourself up.

The issue is that with a single /e, the RHS is understood to be code whose eval’d result is used for the replacement.

What is that RHS? It’s $1. If you evaluated $1, you find that contains the string $var. It does not contain the contents of said variable, just $ followed by a v followed by an a followed by an r.

Therefore you must evaluate it twice, once to turn $1 into $var, then again to turn the previous result of $var into the string "testing". You do that by having the double ee modifier on the s operator.

You can check this pretty easily by running it with one /e versus with two of them. Here’s a demo a both, plus a third way that uses symbolic dereferencing — which, because it references the package symbol table, works on package variables only.

use v5.10;  our $str = q(In this string we are $var the "e" modifier.); our $var = q(testing);  V1: {     local $_ = $str;      s/(\$\w+)/$1/e;     say "version 1: ", $_;  }  V2: {     local $_ = $str;     s/(\$\w+)/$1/ee;     say "version 2: ", $_; }  V3: {     no strict "refs";     local $_ = $str;     s/\$(\w+)/$$1/e;     say "version 3: ", $_; } 

When run, that produces:

version 1: In this string we are $var the "e" modifier. version 2: In this string we are testing the "e" modifier. version 3: In this string we are testing the "e" modifier.