How do I replace an arbitrary number of backreferences in sed or Perl? (for obfuscating mailto)

I'm looking for a way to obfuscate mailtos in the source code of a web site. I'd like to go from this:

href="mailto:president@whitehouse.gov"

To this:

href="" onmouseover="this.href='mai'+'lto:'+'pre'+'sid'+'ent'+'@wh'+'ite'+'hou'+'se.'+'gov'"</code>

I'm probably going to go with a PHP solution instead, like this (that way I only have to globally replace the entire mailto, and the source on my end will look better), but I spent too much time looking at sed and Perl and now I can't stop thinking about how this could be done! Any ideas?

Update: Based heavily on eclark's solution, I eventually came up with this:

#!/usr/bin/env perl -pi
if (/href="mailto/i) {
    my $start = (length $`) +6;
    my $len = index开发者_如何转开发($_,'"',$start)-$start;
    substr($_,$start,$len,'" onmouseover="this.href=' .
    join('+',map qq{'$_'}, substr($_,$start,$len) =~ /(.{1,3})/g));
}

#!/usr/bin/perl

use strict; use warnings;

my $s = 'mailto:president@whitehouse.gov';

my $obfuscated = join('+' => map qq{'$_'}, $s =~ /(.{1,3})/g );

print $obfuscated, "\n";

Output:

'mai'+'lto'+':pr'+'esi'+'den'+'t@w'+'hit'+'eho'+'use'+'.go'+'v'

Note that 'lto: is four characters, whereas it looks like you want three character groups.

Building on Sinan's idea, here's a short perl script that will process a file line by line.

#!/usr/bin/env perl -p

my $start = index($_,'href="') +6;
my $len = index($_,'"',$start)-$start;
substr($_,$start,$len+1,'" onmouseover="this.href=' .
  join('+',map qq{'$_'}, substr($_,$start,$len) =~ /(.{1,3})/g)
);

If you're going to use it, make sure you have your old files committed to source control and change the -p option to -i, which will rewrite a file in place.

Is this close enough?

use strict; 
use warnings; 

my $old = 'href="mailto:president@whitehouse.gov"';
$old =~ s/href="(.*)"/$1/;
my $new = join '+', map { qq('$_') } grep { length $_ } split /(.{3})/, $old;
$new = qq(href=""\nonmouseover="this.href=$new\n");
print "$new\n";

__END__

href=""
onmouseover="this.href='mai'+'lto'+':pr'+'esi'+'den'+'t@w'+'hit'+'eho'+'use'+'.go'+'v'
"

Just an example.

$ echo $s
href="mailto:president@whitehouse.gov"

$ echo $s | sed 's|\(...\)|\1+|g' | sed 's/hre+f=\"/href="" onmouseover="this.href=/'
href="" onmouseover="this.href=+mai+lto+:pr+esi+den+t@w+hit+eho+use+.go+v"

Ack! Thppfft! I offer you this hairball:

s='href="mailto:president@whitehouse.gov"'
echo "$s" | sed -n 's/=/=\x22\x22\n/;
h;
s/\n.*//;
x;
s/[^\n]*\n//;
s/"//g;
s/\(...\)/\x27&\x27+/g;
s/.*/onmouseover=\x22this.href=&\x22/;
x;
G;
s/\n//2;
s/+\([^\x22]\{1,2\}\)\x22$/+\x27\1\x27\x22/;
s/+\x22$/\x22/;
p'