bthe literal b (9810 / 1428 / 6216) (case sensitive)
{4}matches exactly 4 characters from the set below:
0-9one character from 0 (4810) to 9 (5710)
(*SKIP)acts like (*PRUNE), but the next unanchored match attempt starts where (*SKIP) was reached instead of at the next subject character
(*F)forces the current match attempt to fail at this position