here a regex solution which works for the shortest possible tail of length k

  DB<127> $str
 => "abcdabcdabceabcdabcdabceab"

  DB<128> $str=~/^((.+?).*)\2$/; $rest=$1, $tail=$2 
 => ("abcdabcdabceabcdabcdabce", "ab")

  DB<129> $rest =~ /^(.+?)\1*$/; $1
 => "abcdabcdabce"

[download]

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! =)

HTH

Cheers Rolf

( addicted to the Perl Programming Language)

In reply to Re^3: Finding repeat sequences. by LanX
in thread Finding repeat sequences. by BrowserUk

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