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    <p>Deparse tells us this is what is being executed: </p> <pre><code>$find = 'start (.*) end'; $replace = "foo \cA bar"; $var = 'start middle end'; $var =~ s/$find/$replace/; </code></pre> <p>However, </p> <pre><code> /$find/foo \1 bar/ </code></pre> <p>Is interpreted as :</p> <pre><code>$var =~ s/$find/foo $1 bar/; </code></pre> <p>Unfortunately it appears there is no easy way to do this. </p> <p>You can do it with a string eval, but thats dangerous. </p> <p>The most sane solution that works for me was this: </p> <pre><code>$find = "start (.*) end"; $replace = 'foo \1 bar'; $var = "start middle end"; sub repl { my $find = shift; my $replace = shift; my $var = shift; # Capture first my @items = ( $var =~ $find ); $var =~ s/$find/$replace/; for( reverse 0 .. $#items ){ my $n = $_ + 1; # Many More Rules can go here, ie: \g matchers and \{ } $var =~ s/\\$n/${items[$_]}/g ; $var =~ s/\$$n/${items[$_]}/g ; } return $var; } print repl $find, $replace, $var; </code></pre> <h3>A rebuttal against the ee technique:</h3> <p>As I said in my answer, I avoid evals for a reason. </p> <pre><code>$find="start (.*) end"; $replace='do{ print "I am a dirty little hacker" while 1; "foo $1 bar" }'; $var = "start middle end"; $var =~ s/$find/$replace/ee; print "var: $var\n"; </code></pre> <p>this code does exactly what you think it does. </p> <p>If your substitution string is in a web application, you just opened the door to arbitrary code execution. </p> <p>Good Job. </p> <p>Also, it <strong>WON'T</strong> work with taints turned on for this very reason.</p> <pre><code>$find="start (.*) end"; $replace='"' . $ARGV[0] . '"'; $var = "start middle end"; $var =~ s/$find/$replace/ee; print "var: $var\n" $ perl /tmp/re.pl 'foo $1 bar' var: foo middle bar $ perl -T /tmp/re.pl 'foo $1 bar' Insecure dependency in eval while running with -T switch at /tmp/re.pl line 10. </code></pre> <p>However, the more careful technique is sane, safe, secure, <em>and</em> doesn't fail taint. ( Be assured tho, the string it emits is still tainted, so you don't lose any security. )</p>
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    1. POHow to use a variable in the replacement side of the Perl substitution operator?
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    1. USKent Fredric
    1. USKent Fredric
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    1. COThe easy way is ysth's answer. :)
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      1. USbrian d foy
    2. COIt depends on from where the data that's evaluated comes. Avoiding eval is generally a good idea.
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    3. CONo, avoiding eval is not generally a good idea. Using it only with care is.
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