in reply to /e in regexes

The "e" being absent means "treat the replace expression as a double-quote string literal". 

$x = '10+4';
print( "$x" );  # 10+4, not 14

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The "e" being present means "treat the replace expression as a Perl expression".
The Perl expression $1 evaluates to the value of $1, which is the string 10+4. 

$x = '10+4';
print( $x );  # 10+4, not 14

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s/.../.../ee is an awful* way of saying s/.../eval .../e.
That means that not only is $1 treated as a Perl expression, so is the value returned by it. 

$x = '10+4';
print( eval $x );  # 14

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* — In my opinion, something as dangerous as eval EXPR shouldn't be hidden as an simple little "e".

Update: Added illustrative code.

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Re^2: /e in regexes
by waldner (Beadle) on Jun 22, 2008 at 16:44 UTC
    Thanks. But why a single /e is enough in the first case then?
[reply]
      Because in this cases the evaluated string is "10" + 1, which forces perl to treat "10" as a number. If your evaluated string is "10+1", The whole thing is taken as a string literal.
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[d/l]
[select]

      In one case, you want the replacement expression to be a Perl expression,
      In the other, you want the value of $1 to be evaluated. 

      # s/(10)/$1+1/e
$dollar1 = "10";
$replace = $dollar1 + 1;   # One 'e', so Perl expression.
print $replace;            # 11

# s/(10\+4)/$1/e
$dollar1 = "10+4";
$replace = $dollar1;       # One 'e', so Perl expression.
print $replace;            # 10+4

# s/(10\+4)/$1/ee
$dollar1 = "10+4";
$replace = eval $dollar1;  # Two 'e', so Perl expression + eval.
print $replace;            # 14

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      Update: Oops, this was supposed to be a reply to Re^2: /e in regexes.

[reply]
[d/l]
        Thanks everybody. Thanks to this last example it's clear now (I hope).
[reply]

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