<?php

$re = '~^\h*F\d{6}.*?\R{2}\K~ms'; 
$str = "A N K U N F T   11.08.15\n*** NEUBUCHUNG ***\n 11.08.15  xxx  xxx  X3 2830  14:25   17:50\n 18.08.15  xxx  xxx  X3 2831  18:40\n F882129  dsdsaidsaia\n F882129  xxxyxyagydaysd\nsadsdsdsdsadsadadssda\nsadsdsdsdsadsadadssda\n\nA N K U N F T   18.08.15\n*** NEUBUCHUNG ***\n 11.08.15  xxx  xxx  X3 2830  14:25   17:50\n 18.08.15  xxx  xxx  X3 2831  18:40\n F881554  ZXCXZCXCXZCCXZ\n F881554  xcvcxvcxvcvxc\n F881554  xvcxvcxcvxxvccvxxcv\n\n\n11.08.15  xxx  xxx  X3 2830  14:25   17:50\n 18.08.15  xxx  xxx  X3 2831  18:40\n F881605  xczxcdfsfdsdfs\n F881605  zxccxzxzdffdsfds\n\n"; 
print_r(preg_split($re, $str));