I assume that the pattern must repeat at least twice, otherwise, the full string is always the longest answer.

A simple regex can get a good guess and tell you when that guess has failed in such a way that each subsequent guess will be more than twice as long as the previous guess so the regex doesn't have to be run very many times: 

sub repeating {
    my( $string ) = @_;
    my( $pattern, $repeat, $end ) = $string =~ /^(.+?)(\1+)(.*)$/;
    while( defined $pattern ) {
        return $pattern
            if  length($end) <= length($pattern)
            &&  $end eq substr($pattern,0,length($end));
        print "($pattern) wasn't long enough.\n";
        ( $pattern, $repeat, $end ) =
            $string =~ /^(\Q$pattern$repeat\E.+?)(\1+)(.*)$/
    }
    return undef;
}
my $pattern = repeating( "aabaabaabcaabaabaabca" );
printf "(%s) wins\n", $pattern
    if  $pattern;
__END__
(a) wasn't long enough.
(aab) wasn't long enough.
(aabaabaabc) wins

[download]

You likely can optimize this by copying less stuff, of course.

(Update: Well, I didn't get very rigorous in proving to myself that $pattern.$repeat is always too short. But I believe that to be the case. One should validate or refute that assumption before deciding whether to use this.)

- tye        

In reply to Re: Finding repeat sequences. (only mostly regex) by tye
in thread Finding repeat sequences. by BrowserUk

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