/B(?:3[89]|4\d)/
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Thanks much!

CT

(?(?{code}true-regexp)
(?(?{code}true-regexp|false-regexp)

/
   B
   (\d\d)
   (?
      (?{ $1 < 38 || $1 > 49 })  # if invalid
      (?=A)(?=Z)                 # then fail
                                 # else succeed
   )
/x
[download]

It could also have been written:

/
   B
   (\d\d)
   (?(?{ $1 >= 38 && $1 <= 49 })  # if valid
                                  # succeed
      |                           # else
      (?=A)(?=Z)                  # fail
   )
/x
[download]

(?=A)(?=Z) means the next char must be an 'A' and must be a 'Z'. That's never gonna happen.