Get item of index with strings

Hi everyone,
If I have a list of strings (for example {A1 A2 A3 B1 B2 B3 C1 C2 C3}, how can I get the index of strings that start with B? (index of B1 B2 B3)
Thanks a lot.

Find all elements that start with “B” then get their indices:

Thanks a lot Habdirad.