Re^2: Regular expression "replace string interpolation" problem

[cLive ;-)]

in reply to Re: Regular expression "replace string interpolation" problem
in thread Regular expression "replace string interpolation" problem

That works - thanks :)

But, reading through the regex documentation, I still can't work out exactly why. Hmmm.

so I guess needs to be \\" in the replace string for quotes. Arghhh, double escaping!!

[ikegami]

Or how to embed " in the output - but that's not a biggie right now.

You want to produce the code

"$1 \"Perl\""

so

my $replace = '"$1 \\"Perl\\""';  # Produces: "$1 \"Perl\""
s/$match/eval $replace/e

Technically, the slashes don't need to be doubled since single quotes are forgiving when it's unambiguous.

[pc88mxer]

To explain what's going on it might be helpful to use underlined bold text to represent strings. So, one can write:

eval 2+2 → 4

and this means if you eval the string 2+2, you'll get the string 4. In perl this is just:

my $x = '2+2';
my $y = eval $x; # $y is the string '4'

In your original example, what is happening during the substitution s/$match/$replace/e is:

eval $replace → $1 Perl

I.e., the match is replaced with the string $1 Perl, and that is why $text2 contains $1 Perl world.

How about just adding another /e modifier to evaluate the substitution again? Unfortunately this doesn't work because $1 Perl world is not valid perl syntax:

eval eval $replace → eval $1 Perl world → syntax error

When $replace and the substitution is written as:

my $replace = '"$1 Perl"';
$text2 =~ s/$match/$replace/ee;

the evaluation of the replacement proceeds as follows:

eval eval $replace → eval "$1 Perl"

and because of the double quotes this last eval yields $1 concatenated with a space and the string Perl.