Convert Pascal Case to Snake Case with Python and Regex.

pascal_to_snake.py

import re

def to_snake(pascal:str) -> str:
  """Converts a Pascal case string to snake case.
  """
  magic = re.findall('[A-Z]+[a-z]*', pascal)
  snake = '_'.join(magic)
  snake = snake.lower()
  
  return snake
 
# Working examples:
assert to_snake("ID") == "id"
assert to_snake("HelloWorld") == "hello_world"
assert to_snake("EndsWithAllCapsXML") == "ends_with_all_caps_xml"

# Broken Examples:
assert to_snake("STARTINGWithAllCapsDoesntWork") == "startingwith_all_caps_doesnt_work"
assert to_snake("startingWithLowerCaseDoesntWork") == "with_lower_case_doesnt_work"

I wrote this because the other regex examples I found in this StackOverflow question didn't capture the right scenarios, particularly ID to id, and a string ending with all caps (XML).