'b' and 'B' on second digit of number returns to incorrect place

Using the following tests, the back command can be shown to behave inappropriately. ^ represents the cursor position.
```
word 8888.8888
      ^
b
word 8888.8888
^
```
With B
```
word 8888.8888
      ^
B
word 8888.8888
^
```

Correct behavior would yield this:
```
word 8888.8888
      ^
b
word 8888.8888
     ^
```

Thank you!