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  1. or download this
    /^(.{4}).*\1$/     # ^ and $ match beginning and end of string respect
+ively
  2. or download this
    /^(.{4}).*\1.*\1$/
  3. or download this
    ABCD # pattern

...
ABED # match
BBCD # match
ABC  # no match
  4. or download this
    sub makepattern
{
...
my $imprecisematch = "ABCD";
my $pattern = makepattern( $imprecisematch );
/^($pattern).*\1.*\1$/     #match $_ against three occurrences of this
+ pattern