regexp confusion

I can't tell if this is a bug or not. I'm using ACL 6.2 with all recent 
patches applied.

CL-USER(19): (match-regexp "^[0-9 ,]*$" "4, 7")
T
"4, 7"
CL-USER(21): (match-regexp "^[0-9\\b,]*$" "4, 7")
NIL

The only difference is that in the regexp, I substitute a space for \\b, 
and all of a sudden the string matches. Isn't \\b supposed to be a 
catch-all for all kinds of whitespace?

-JT