Yow. I'm not sure who's crazier - you for suggesting this might be something one would want to do, or me for trying to do it ;)

Because It's There, as the man said.

Having struggled with it a bit I realised one thing about the question itself, which is that we can't say there are only three patterns. In fact there are a lot more - "hell", "hel", and "he" to name but the most obvious additions. That's unless we want to match against a dictionary, in which case it's just a matter of processing power.

Assuming we are interested in patterns rather than specific words I think the following does it. I should say at the outset that the clever bit in this comes from japhy's regex book which is referred to in this node.

my $string = "helloworldhellohellohihellohiworld";
my $length = length $string;
my $window = int (($length - 2) / 2);


# use japhy's regex to hoover up all char
# sequences that MIGHT be patterns:

my @pats;
my $regex;
while ($window > 1) {
   $regex = '(?=(' . '.' x $window . '))';
   push @pats, ($string =~ /$regex/g);
   $window --;
}


# now go through @pats to find the duplicates
# and print the final result

@pats = sort @pats;
my %dups;
for (2 .. $#pats) {
    $dups{$pats[$_]} ++ if ($pats[$_] eq $pats[$_ - 1])
}
$dups{$_} ++ for keys %dups;
for (keys %dups) {print $dups{$_},' occurrences of "',$_,'"',"\n"}
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This throws up 31 patterns, with up to four occurrences each. (BTW, in case $window doesn't make sense, I assumed (A) there must be at least two occurrences of each pattern, otherwise it wouldn't really be a pattern; (B) each pattern must be at least 2 chars and (C) there must be at least 2 patterns.)

Thanks for making me think. Can I stop now?

§ George Sherston