Get item of index with strings

Lists-Logic
1

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.

2

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

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3

Thanks a lot Habdirad.