cap per run: 2000 ms; input = n letters 'a' followed by '!' (no match possible)

pattern (a+)+b
  n=20     0 ms  matches=false
  n=36     0 ms  matches=false
  n=500    1 ms  matches=false
  n=1000   23 ms  matches=false
  n=2000   63 ms  matches=false
  n=4000   161 ms  matches=false

pattern (a+)+\1?b
  n=20     27 ms  matches=false
  n=24     690 ms  matches=false
  n=26     ABORTED after 2036 ms (cap)

pattern (?:(?:a+)+)+b
  n=16     3 ms  matches=false
  n=18     13 ms  matches=false
  n=20     101 ms  matches=false
  n=22     896 ms  matches=false
  n=24     ABORTED after 2000 ms (cap)

pattern a*a*a*a*b
  n=50     2 ms  matches=false
  n=100    69 ms  matches=false
  n=150    285 ms  matches=false
  n=200    824 ms  matches=false