Help with Regexp substitution using variables

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

Hi, I'm trying to write an application that takes 2 regular expressions as input, and then do a substitution on a file. The problem happens when I try to use the memory variable as the second substitution. The resulting output becomes the literal '$1' as opposed to the match in parenthesis. It's better to look at the code.

my ($reg, $reg2) = @ARGV;
my $string = "how1=on&how2=on";

$string =~ s/$reg/$reg2/eg;
print $string;
[download]

Input: perl test.pl how(\d)=on why$1=off
What I want the ouput to be is: why1=on&why2=on
What I actually get is: why$1=off&why$1=off.
Any help is greatly appreciated!
Thanks,
Nathan

Comment on Help with Regexp substitution using variables Download Code

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Re: Help with Regexp substitution using variables by ikegami (Patriarch) on Jan 20, 2005 at 02:48 UTC
The following works, but you risk injecting commands into your script. To do it properly, you'd have to do the variable interpolation yourself. `my ($reg, $reg2) = @ARGV; my $string = "how1=on&how2=on"; $string =~ s/$reg/ eval "\"$reg2\"" /eg; print $string;` [download]	[reply] [d/l]
Re^2: Help with Regexp substitution using variables by Roy Johnson (Monsignor) on Jan 20, 2005 at 17:41 UTC
Or `$string =~ s/$reg/qq{qq{$reg2}}/eeg;` [download] Caution: Contents may have been coded under pressure.	[reply] [d/l]
Re^3: Help with Regexp substitution using variables by ikegami (Patriarch) on Jan 20, 2005 at 19:26 UTC
I like making the `eval EXPR` evident over adding a second /e, because of the inheritant risks to using them.	[reply] [d/l]
Re: Help with Regexp substitution using variables by Ardemus (Beadle) on Jan 20, 2005 at 04:13 UTC
I don't have have an answer, but I can give you some information to make it easier to find. You need recursive variable interpolation. The /e in your regex stands for eval. It executes the result as perl code (see taint checking if you want to start looking into security). Naturally, the evaluation of $reg2 is its contents. You can use /ee to eval results of the first eval (as Dave suggested). In fact, I believe that you can add 'e's to recursively evaluate as many iterations as you want. For example: `> perl -e "$string='ReplaceMe';$s1='Bug';$s2='$s1';$s3='$s2';$string=~ +s/Me/$s3/eeeg;print $string" > ReplaceBug` [download] I hope this helps.	[reply] [d/l]
Re: Help with Regexp substitution using variables by kwokna (Initiate) on Jan 20, 2005 at 17:02 UTC
Wow, thanks for the quick replies, everyone! I didn't know you could keep adding 'e' to a reg. pretty cool	[reply]
Re: Help with Regexp substitution using variables by dave_the_m (Monsignor) on Jan 20, 2005 at 02:50 UTC
`$string =~ s/$reg/'"'.$reg2.'"'/eeg;` [download] is one of many possibilities Dave.	[reply] [d/l]