here a regex solution which works for the shortest possible tail of length k
DB<128> $str=~/^((.+?).*)\2$/; $rest=$1, $tail=$2
=> ("abcdabcdabceabcdabcdabce", "ab")
DB<129> $rest =~ /^(.+?)\1*$/; $1
needs to be extended for longer possible tails.
But taking the dimensions of your data I doubt that regexes are appropriate.
You could test all $patterns which repeat at least once (or x times) and calculate $k = $m % $l with $m =length ($str), and check if $str starts and ends with the same substring $tail of length $k and then check if the pattern continues repeating.
Or start eliminating all possible $tails and check if $l of a repeating pattern is a divisor of the $rest.
Had no time to check all the other posted solutions and don't wanna reinvent the wheel, so I better stop here! =)
( addicted to the Perl Programming Language)
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