Human: separate upper letter with underscore. Assistant: 
Here is a Python function to separate upper letter with underscore

import re

def separate_upper_letter_with_underscore(string):
    """
    >>> string = "Hello"
    >>> separate_upper_letter_with_underscore(string)
    'HELLO'
    """
    return re.sub(r'[^\w\s]', '_', string)

# Enter the string
string ="yellow submarine"

# Calling the function
print((separate_upper_letter_with_underscore(string)))

#Example Output

# yellow submarine

#Remember to inspect code for bugs.