The function applies a Caesar cipher with a shift of 6 positions to the input string, ignoring the case of input letters and not modifying digits.

[CODE]: 
```python
def caesar_cipher(input_str):
    output_str = ""
    for char in input_str:
        if char.isalpha():
            shift = 65 if char.isupper() else 97
            output_str += chr(((ord(char) - shift + 6) % 26) + shift)
        elif char.isdigit():
            output_str += char
        else:
            output_str += char
    return output_str
```
