What is the process of using Python to isolate letters from a string containing both letters and numbers?

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1. Start by importing the re module to use regular expressions.
2. Use the re.sub() function to replace all non-letter characters with a space.
3. Use the split() function to split the string into a list of words.
4. Use a loop to iterate through each word in the list.
5. Use the isalpha() function to check if the word contains only letters.
6. If the word contains only letters, append it to a new list.
7. At the end of the loop, join the letters in the new list into a string.

Here is an example code snippet:

import re

string_with_letters_and_numbers = "a1b2c3d4"

# Replace non-letters with spaces
letters_only = re.sub('[^a-zA-Z]', ' ', string_with_letters_and_numbers)

# Split into a list of words
words = letters_only.split()

letters = []
for word in words:
    if word.isalpha():
        letters.append(word)

# Join the letters into a string
result = ''.join(letters)

print(result) # Output: abcd