Re: How to do regex backreferences within $variable replacement text?

Here's some example code on how to do this with a few tests. I'd invite adding some more tests and any bug reports/security issues as I've not really thought about this from a security perspective yet. It's a bit slower in that it matches twice, but it completely avoids any eval.

use strict;
use warnings;

sub substitute
{
    my ($string, $from, $to) = @_;

    $from = qr/$from/ unless ref $from and ref $from eq 'Regexp';
    my @a = $string =~ $from;
    $to =~ s/\$(\d+)/$a[$1-1]/g; # was $to =~ s/\$(\d+)/\Q$a[$1-1]/g;
    $string =~ s/$from/$to/;
    $string;
}

my @tests = (
             [ "this is some test", "(is) s(o)me", '$1 n$2t a' ],
             [ "this is some test", "is some",   'is not a' ],
            );

for my $t (@tests)
{
    print "[$t->[0]]...";
    print "[",substitute(@$t), "]\n";
}
[download]

prints out:

[this is some test]...[this is not a test]
[this is some test]...[this is not a test]
[download]

which is what I expected. But, as you can see, it's not a very extensive test, so feel free to try a few more.

Update: It turns out that the \Q in the $to replacement wasn't needed.

Comment on Re: How to do regex backreferences within $variable replacement text? Select or Download Code

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Re^2: How to do regex backreferences within $variable replacement text? by ManFromNeptune (Scribe) on Sep 18, 2005 at 03:12 UTC
Ok, this appears to be working, except for one thing: for the backreferenced sections, spaces are getting prepended with a backslash in the $to clause, and subsequently in the $string. Here's a test I added: `[ 'Once upon a time, Jack Roush was not the king of the NASCAR garage, + but a stock-car outsider from Michigan trying to start a Winston Cup + team with a small-time budget.', '(king of the NASCAR )(garage, but +a stock-car)', 'HERE1$1HERE2$2HERE3' ]` [download] And here's the output: `[Once upon a time, Jack Roush was not the king of the NASCAR garage, b +ut a stock-car outsider from Michigan trying to start a Winston Cup t +eam with a small-time budget.] ... [Once upon a time, Jack Roush was not the HERE1king\ of\ the\ NASCAR\ +HERE2garage\,\ but\ a\ stock\-carHERE3 outsider from Michigan trying +to start a Winston Cup team with a small-time budget.]` [download] Any ideas? I can't see where these backslashes are coming from...!	[reply] [d/l] [select]
Re^2: How to do regex backreferences within $variable replacement text? by ikegami (Patriarch) on May 10, 2006 at 21:57 UTC
When allowing the emdedding of code (loosely defined), provide an escape mechanism!!! For example, I have no means of replacing with `$1 . "00"`. Perl uses `"${1}00"`. I have no means of replacing with `'$1.00'`. Perl uses the literal `"\$1.00"`. When adding your escape mechanism, careful not to break existing functionality. For example, Continue allowing me to replace with `'\$1'`. Perl uses `"\\$1"`. Update: A solution is to replace `$to =~ s/\$(\d+)/$a[$1-1]/g;` [download] with `$to =~ s/\\(.)\|\${(\d+)})\|\$(\d+)/ (defined $1 ? $1 : (defined $2 ? $a[$2-1] : $a[$3-1] ) ) /eg;` [download]	[reply] [d/l] [select]