Can't wrap my head around how to remove a list of characters from another list

I've been able to isolate the list (or string) of characters I want excluded from a user entered string. But I don't see how to then remove all these unwanted characters. After I do this, I think I can try joining the user string so it all becomes one alphabet input like the instructions say.

Instructions:

Remove all non-alpha characters Write a program that removes all non-alpha characters from the given input.

For example, if the input is:
-Hello, 1 world$!
the output should be:
Helloworld

My code:

userEntered = input()
makeList = userEntered.split()
def split(userEntered):
    return list(userEntered)
    
    
if userEntered.isalnum() == False:
    for i in userEntered:
        if i.isalpha() == False:
            #answer = userEntered[slice(userEntered.index(i))]
           reference = split(userEntered)
           excludeThis = i
           print(excludeThis)

When I print excludeThis, I get this as my output:

-
,
 
1
 
$
!

So I think I might be on the right track. I need to figure it out how to get these characters out of the user input. Any help is appreciated.

Solution

Loop over the input string. If the character is alphabetic, add it to the result string.

userEntered = input()
result = ''
for char in userEntered:
    if char.isalpha():
        result += char
print(result)

This can also be done with a regular expression:

import re

userEntered = input()
result = re.sub(r'[^a-z]', '', userEntered, flags=re.I)

The regexp [^a-z] matches anything except an alphabetic character. The re.I flag makes it case-insensitive. These are all replaced with an empty string, which removes them.