Regexp variable substitution of special variable

cormanaz has asked for the wisdom of the Perl Monks concerning the following question:

Good afternoon monks. I am trying to do a variable substitution in a regexp that contains a reference to a special variable and it's not working. When I try to do the same substitution directly it works, as illustrated in this simplified example (in reality I'm trying to do this with many different search/replace strings):

$string = "firstword secondword";
print "original: $string\n";
# doesn't work
$search = '(second.+)';
$replace = '$1 thirdword';
($varsub = $string) =~ s/$search/$replace/;
print "variable substitution: $varsub\n";
# does work
($direct = $string) =~ s/(second.+)/$1 thirdword/;
print "direct: $direct\n";
[download]

How do I get perl to recognize the special variable in the $replace string?

Steve

Comment on Regexp variable substitution of special variable Download Code

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Re: Regexp variable substitution of special variable by davido (Cardinal) on Jun 30, 2005 at 22:24 UTC
`$replace = '$1 thirdword'; ($varsub = $string) =~ s/$search/$replace/;` [download] Should be replaced with... `$replace = '$1 . " thirdword"'; ($varsub = $string) =~ s/$search/$replace/ee;` [download] ...at least that is if you are looking for a solution that is pretty close to what you're attempting. Dave	[reply] [d/l] [select]
Re^2: Regexp variable substitution of special variable by cormanaz (Deacon) on Jul 01, 2005 at 00:23 UTC
Both this and diotalevi's code works. Thanks for the suggestions. Is one method superior to the other?	[reply]
Re: Regexp variable substitution of special variable by diotalevi (Canon) on Jun 30, 2005 at 22:19 UTC
Write your replacement this way instead: `s/$search/eval qq["$replace"]/e`. The key is the /e and the eval using a bit of runtime constructed perl code. Thanks to davido for noticing a bug that would prevent my original suggestion from working.	[reply] [d/l]
Re^2: Regexp variable substitution of special variable by Tanktalus (Canon) on Jul 01, 2005 at 18:01 UTC
Another minor comment ... e's stack. `s/$search/qq["$replace"]/ee` is exactly the same as yours. Which is better is a matter of opinion. :-)	[reply] [d/l]
Re^3: Regexp variable substitution of special variable by diotalevi (Canon) on Jul 01, 2005 at 21:09 UTC
I think its better to make the second eval explicit and very visible because it is a very different sort of eval from the first /e. So I really, strongly, prefer my version. They're functionally equivalent but you can see that something really, highly unusual is happening in mine. It isn't so visible in yours.	[reply]