How to match two arrays

Question

I have two arrays

A = [a, b, c, d]

and

B = [a1, a2, b1, b2, b3, c1, c2, c3, d1, d2, d3, d4]

I want to match b

Answer 1

from collections import defaultdict

A = ["a", "b", "c", "d"]
B = ["a1", "a2", "b1", "b2", "b3", "c1", "c2", "c3", "d1", "d2", "d3", "d4"]
d = defaultdict(list)
for item in B:
    prefix = item[0]
    if prefix in A:
        d[prefix].append(item)