The function extracts the digits from the input string and converts them into words.

[CODE]: 
```python
def f(x):
    digit_to_word = {
        '0': 'zero', '1': 'one', '2': 'two', '3': 'three', '4': 'four',
        '5': 'five', '6': 'six', '7': 'seven', '8': 'eight', '9': 'nine'
    }
    output = ' '.join([digit_to_word[digit] for digit in x if digit.isdigit()])
    return output
```