<human>:  Give me a Python code to replace all occurences of old with new in a string.
<bot>: Here is a Python code to replace all occurences of old with new in a string

import re

def replace_all(old, new, string):
    """
    Replaces all occurences of old with new in a string.
    """
    return string.replace(old, new)

# Enter the string
string = "This is a string"

# Calling the function
print((replace_all("a", "b", string)))

#Example Output
# This is b string

#Remember to inspect code for bugs.