m{
  A         # A
  (?:
    [^AB]*  # 0 or more non-A and non-B characters
    . B     # any character, then a B
  )*        # this combo, 0 or more times
  [^AB]*    # 0 or more non-A and non-B characters
  A         # an A
}x

##</code><code>##

BEFORE & AFTER       REGEX
<>  <A01234567890A>  A
<A>  <01234567890A>  [^AB]*
<A01234567890>  <A>  .
<A01234567890A>  <>  B       FAILED
<A0123456789>  <0A>  .
<A01234567890>  <A>  B       FAILED

##</code><code>##

<A012345678>  <90A>  .
<A0123456789>  <0A>  B       FAILED

##</code><code>##

<A01234567890>  <A>  [^AB]*
<A01234567890>  <A>  A
<A01234567890A>  <>          FINISHED