I would like to replace strings with the number of times that a `B` character is repeated in the string.

This is the representative input `df`:

```
df <- c("AA", "AB", "BB", "BBB", "D", "ABB")

```

My expected `out` output would be like that:

```
out <- c("0", "1", "2", "3", "0", "2")

```

Any idea? Thank you!