It attemps to match and backtracks when it can't find a match. Here's a sample of the matches (one per line) the regexp engine attempts.

1)  a*->"a"x61, a*->"",   a*->"",   a*->"",  ... a*->"",  [b]->fail
2)  a*->"a"x60, a*->"a",  a*->"",   a*->"",  ... a*->"",  [b]->fail
3)  a*->"a"x60, a*->"",   a*->"a",  a*->"",  ... a*->"",  [b]->fail
4)  a*->"a"x60, a*->"",   a*->"",   a*->"a", ... a*->"",  [b]->fail
.
.
.
32) a*->"a"x60, a*->"",   a*->"",   a*->"",  ... a*->"a", [b]->fail
33) a*->"a"x60, a*->"",   a*->"",   a*->"",  ... a*->"",  [b]->fail
34) a*->"a"x59, a*->"aa", a*->"",   a*->"",  ... a*->"",  [b]->fail
35) a*->"a"x59, a*->"a",  a*->"a",  a*->"",  ... a*->"",  [b]->fail
36) a*->"a"x59, a*->"a",  a*->"",   a*->"a", ... a*->"",  [b]->fail
.
.
.
64) a*->"a"x59, a*->"a",  a*->"",   a*->"",  ... a*->"a", [b]->fail
65) a*->"a"x59, a*->"a",  a*->"",   a*->"",  ... a*->"",  [b]->fail
66) a*->"a"x59, a*->"",   a*->"aa", a*->"",  ... a*->"",  [b]->fail
67) a*->"a"x59, a*->"",   a*->"a",  a*->"a", ... a*->"",  [b]->fail
.
.
.
95) a*->"a"x59, a*->"",   a*->"a",  a*->"",  ... a*->"a", [b]->fail
.
.
.
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The character class [b] is necessary because a plain b would trigger optimizations in the regexp engine. The optimization would cause it to scan for the location of b chars in the string to match. Finding none, it would exit immediately.