parameter expansion, substitution

["S. Cowles" <scowles@ckhb.org>]

i am not getting expected results in the following code snippet:
 	b="abc" ; b=${b/#abc%/d} ; echo ${b}
i expect the value of b to be changed to "d"; however, it remains "abc".

the following snippet does give the expected result:
 	b="abc" ; b=${b/#abc/d} ; echo ${b}

for this session, the following shell options are set: interactive monitor 
shinstdin zle.  the version of the shell being used is 4.3.17-dev-0, 
patchlevel 1.5604.

in my use case, i need to specify a pattern match at the end of the 
parameter.  have i misunderstood the syntax of the '%' end-of-string 
operator?  or is this problem something else?

thanks for any help.