s/RegEx/substitutions/: Variable interpolation and when to use /e - modifiers

[LanX]

Let me try to write a deeper conceptional answer, because the handling of /e irritated me for long and I need to pin down the source of this irritation myself.

The short answer

The RHS of s/LHS/RHS/ undergoes a variable interpolation (like doublequotes) if and only if there are no /e modifiers used.

modifiers	substition	equivalent
no e	VAR =~ s/LHS/RHS/	VAR =~ m/LHS/; MATCH = "RHS"
one e	VAR =~ s/LHS/RHS/e	VAR =~ m/LHS/; MATCH = eval 'RHS'
two e	VAR =~ s/LHS/RHS/ee	VAR =~ m/LHS/; MATCH = eval eval 'RHS'
more e	Don't! Only for brainfuck and obfuscation, better call function

MATCH meaning the substring from VAR matching the LHS-pattern.

For completeness nota bene ...

concept	code	equivalent
disabling interpolation	VAR =~ s'LHS'RHS'	VAR =~ m/LHS/; MATCH = 'RHS'
string interpolation	$a = "$b - $c"	$a = $b . ' - ' . $c
variable evaluation	$a = $b	$a = eval '$b'

The long answer ...

Problem

At first it's confusing that there is no difference between s/(pattern)/$1/ and s/(pattern)/$1/e

example:

  DB<1> $_='abc'; s/a(b)c/$1/; print $_
b
  DB<2> $_='abc'; s/a(b)c/$1/e; print $_
b

Explanation

Line 1 w/o e-modifier but with interpolation:

Variables in RHS undergo an interpolation like the result in double-quotes "RHS"!

the following code is equivalent

  DB<1> $_='abc'; s/a(b)c/$1/; print $_
b
  DB<3> $_='abc'; m/a(b)c/; $_ = "$1" ; print $_
b

Line 2: with e-modifier but w/o interpolation:

RHS is evaluated without variable interpolation¹ like in simple quotes eval 'RHS'

  DB<2> $_='abc'; s/a(b)c/$1/e; print $_
b
  DB<4> $_='abc'; m/a(b)c/; $_= eval '$1'; print $_
b

And $_= eval '$1' just means $_= $1

I hope now double evals /ee can be better understood

Like ikegami stated correctly those  s/LHS/RHS/ee; and  s/LHS/eval RHS/e; are equivalent

Those double /ee are used if you need a matchvariables (a substring from LHS) to be interpolated within an eval, b/c variable interpolation is missing!

Any code from LHS will be treated as uninterpolated string, as in these equivalent expamples

  DB<5> $_='warn 666'; s/(.*)/$1/e; print $_
warn 666
  DB<6> $_='warn 666'; m/(.*)/; $_= eval '$1'; print $_
warn 666

So if I need code-evaluation from parts of LHS I need a second /e

 
  DB<7> $_='warn 666'; s/(.*)/$1/ee; print $_
666 at (eval 171) ....    # skipped error message
1      # $_ = return code from warn
 
  DB<8> $_='warn 666'; m/(.*)/; $_= eval eval '$1';  print $_
666 at (eval 175). ... # skipped error message
1      # $_ = return code from warn

Resumee

I think this explanation could be better worded ...

It already helped me a lot to clarify whats happening and I hope it helps you too or anyone else who tries to explain it better. =)

1) As a side note:

Variable interpolation is a powerful tool which allows some (not all) "evaluation like effects" like interpolation of hash-variables.

  DB<134> $hash{b}=666;
 
  DB<135> $_='abc'; s/a(b)c/$hash{$1}/; print $_
666
  DB<136> $_='abc'; m/a(b)c/; print "$hash{$1}";
666