How does bracket expression and negation [^ ] work with grep ?

commandline session

$ 4.2.20 19 519---> grep '[^b  ]sort' worked
Heapsort
Quicksort
 Timsort
$ 4.2.20 20 520---> cat worked
Bubble sort
Selection sort
Insertion sort
Shell sort
Comb sort
Merge sort
Heapsort
Quicksort
Counting sort
 Bucket sort
 Radix sort
 Distribution sort
 Timsort
$ 4.2.20 21 521--->