(a)?(b)? match problem

[homokia]

I think ab should matches (a)?(b)?. But it doesn't. If the regexp is a?b? then it is ok.

========================> [^(a)?(b)?$] [ab]
SETTING BRACKET 1 [a]
SETTING BRACKET 2 [b]
0 0 [^(a)?(b)?$] [ab]
[^(a)?(b)?$] [ab] re_len=10 step=1 i=0 j=0
[(a)?(b)?$] [ab] re_len=10 step=3 i=1 j=0
QUANTIFIER: [(a)]? [ab]
[(a)] [ab] re_len=3 step=3 i=0 j=0
CAPTURING [(a)] [ab] [?(b)?$]
1 0 [a] [ab]
[a] [ab] re_len=1 step=1 i=0 j=0
<- 1
CAPTURED [(a)] [ab]:1
[(b)?$] [b] re_len=10 step=3 i=5 j=1
QUANTIFIER: [(b)]? [b]
[(b)] [b] re_len=3 step=3 i=0 j=0
CAPTURING [(b)] [b] [?$]
1 0 [a] [b] // I think here should be 1 0 [b] [b]
[a] [b] re_len=1 step=1 i=0 j=0
<- -1
CAPTURED [(b)] [b]:-1
[$] [b] re_len=10 step=1 i=9 j=1
<- -1