What are the difference between `\b` vs `\S` in PCRE grep?

I know now that \b is for Word Boundaries while \s and \S is for Shorthand Character Classes where \s (lowercase) is for whitespace (space, tab, vtab, newline) and \S (uppercase) is the other way around. Anything but whitespace wolf@linux:~$ cat space.txt 0space 1 spaces 2 spaces 3 spaces 4 spaces wolf@linux:~$ I don't see any difference in these 2 commands. wolf@linux:~$ grep -P '\S\s\S' space.txt 1 spaces wolf@linux:~$ grep -P '\S\s\S\S' space.txt 1 spaces wolf@linux:~$ vs wolf@linux:~$ grep -P '\b\s\b' space.txt 1 spaces wolf@linux:~$ grep -P '\b\s\b\b' space.txt 1 spaces wolf@linux:~$ Reference: https://staff.washington.edu/weller/grep.html