$pattern = 'AB';
'BABABABBB' =~
/
   ^                   # start at beginning of line
   (                   # capture to $1
      .*?                 # a number of character, but as few as possible ...
      (?<!$pattern)       # ... which may not contain $pattern
   )
   (                   # capture to $2
      (?:$pattern)+       # multiple occurences of $pattern
         |                   # OR
      (?!.*?$pattern)     # nothing, BUT there may be no $pattern in the rest
                          #    of the string
   )
   (.*)                # capture rest to $3
/x ;
print "$1<$2>$3$/";
__END__
prints "B<ABABAB>BB"