Match only A or B case insensitive

Question

I want a regular expression that validates a string of length 1 which has either the characters A or B (case insensitive) at the Begin

So this should return true

  A
  a
  B
  b

Any word would not match, so the following would return false:

 "America"
 "Bi"
 "a door"

Answer 1

You can use /^[AaBb]$/. That's the simplest I know.