Re: Try this in Regexp palindrome

Using a delayed execution assertion, typically used to create recursively matching regexes, I came up with this:

my $re;
$re = qr/((.)(??{ $re })\2|.)/;
my $s = 'abcdegedfc';
foreach my $i (0 .. length($s) - 1) {
    local $\ = "\n";  # add newline to print
    substr($s, $i) =~ /^$re/ and print $1;
}
[download]

I need a loop, because the regex by itself doesn't search for the longest palindrome, so I added a /^/ anchor to avoid any potential, useless backtacking. It doesn't actually chnage the result.

This little program prints:

a
b
c
deged
ege
g
e
d
f
c

So all you still have to do, is keep the longest match from the loops.

update: I found a second working regex, inspired by holli who came up with the idea to use reverse:

my $s = 'abcdegedfc';
foreach my $i (0 .. length($s) - 1) {
    local $\ = "\n";  # add newline to print
    substr($s, $i) =~ /((.*).?(??{ quotemeta reverse $2 }))/
      and print $1;
}
[download]

Output is identical to the previous program.

Comment on Re: Try this in Regexp palindrome Select or Download Code

Replies are listed 'Best First'.
Re^2: Try this in Regexp palindrome by Roy Johnson (Monsignor) on Mar 21, 2005 at 14:18 UTC
Your delayed-evaluation expression isn't quite right. It won't match palindromes like 'feef' that don't have a central character. You need to make the single dot optional. And if you put the alternation in the middle, you won't match one-character "palindromes". You can also simplify your search by putting the pattern in a lookahead and doing a while-global match: `$_ = 'abcdegedfc'; # Try it without the g, too. my $re; $re = qr/((.)(?:(??{$re})\|.?)\2)/; print "$1\n" while (/(?=$re)/g);` [download] If you change the * to a + in the `reverse` version, you exclude single-character palindromes. The lookahead while-global trick helps here, too: `$_ = 'abcdegedfc'; print "$1\n" while (/(?=((.+).?(??{quotemeta reverse $2})))/g);` [download] Caution: Contents may have been coded under pressure.	[reply] [d/l] [select]
Re^3: Try this in Regexp palindrome by bart (Canon) on Mar 21, 2005 at 17:21 UTC
Your delayed-evaluation expression isn't quite right. It won't match palindromes like 'feef' that don't have a central character. You need to make the single dot optional. You're right. I knew that, and I still forgot about it. Thanks for the correction.	[reply]


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