Using scalar variables as substitution regular expression

addy7de has asked for the wisdom of the Perl Monks concerning the following question:

Hi, I want to use scalar variables which store regular expressions while using string substitution. For Example:

$_ = "master442arb_ba";
$rule = "master\(.*\)arb_ba";
$action = "m\$1a_ba";
s/$rule/$action/;
#s/master(.*)arb_ba/"m${1}a_ba"/;
print $_;
[download]

The correct output is "m442a_ba". The commented s/// command gives the correct
output but if I try the same thing with the $rule and $action variables,
it gives "m$1a_ba" as the output. Can someone explain why and what would be the correct expression?

Thanks!

Comment on Using scalar variables as substitution regular expression Download Code

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Re: Using scalar variables as substitution regular expression by Anonymous Monk on Apr 05, 2009 at 08:46 UTC
1) Use code tags, never pre tags, Markup in the Monastery 2) Can someone explain why and what would be the correct expression? Its basic "interpolation". Interpolating backreferences in an eval, reinterpolation of regexp (and strings?), Evaluating $1 construct in literal replacement expression,	[reply]
Re: Using scalar variables as substitution regular expression by baxy77bax (Deacon) on Apr 05, 2009 at 08:44 UTC
or maybe this would be easier: `$_ = "master442arb_ba"; $_ =~ /master(.*)arb_ba/; $action = "m$1a_ba"; s/$_/$action/; print $_;` [download]	[reply] [d/l]
Re^2: Using scalar variables as substitution regular expression by almut (Canon) on Apr 05, 2009 at 09:44 UTC
... in which case you don't need the substition at all :) `$_ = "master442arb_ba"; $_ =~ /master(.*)arb_ba/; $_ = "m$1a_ba"; print $_; # m442a_ba` [download] (the `s/$_/$action/` in your snippet looks for "master442arb_ba" in "master442arb_ba" and replaces this (i.e. itself) with "m442a_ba" — an assignment is shorter...)	[reply] [d/l] [select]
Re: Using scalar variables as substitution regular expression by addy7de (Initiate) on Apr 05, 2009 at 08:35 UTC
Got It! we can do it with "eval" as follows: $subst = "s/${rule}/${action}/"; eval ($subst) or die;	[reply]
Re^2: Using scalar variables as substitution regular expression by Anonymous Monk on Apr 05, 2009 at 08:49 UTC
Don't forget to read perlsec	[reply]