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"

⚠️ DO NOT USE OPs IMPLEMENTATION ⚠️
Careful with this implementation, one would assume to_snake() to be idempotent, but to_snake("already_snake") == ''.

Don't shoot yourself in the foot here. You can find an improved version below. It handles idempotency, uppercases and camel cases correctly (afaik).

👁️ I'm unaware that lower case letters should be discarded when converting from PascaslCase to snake_case, i. e. assert to_snake("startingWithLowerCaseDoesntWork") == "with_lower_case_doesnt_work" - so the below implementation does NOT do that

import re

def to_snake(s: str) -> str:
    # Add an underscore before each uppercase letter that is followed by a lowercase letter
    s = re.sub(r'([A-Z]+)([A-Z][a-z])', r'\1_\2', s)
    # Add an underscore before each lowercase letter that is preceded by an uppercase letter
    s = re.sub(r'([a-z\d])([A-Z])', r'\1_\2', s)
    # Convert the entire string to lowercase
    s = s.lower()
    return s

# Example usage
print(to_snake("PascalCase"))  # Output: pascal_case
print(to_snake("pascal_case")) # Output: pascal_case # works, idempotent
print(to_snake("camelCaseExample"))  # Output: camel_case_example
print(to_snake("SomeID"))  # Output: some_id 
print(to_snake("ThisIsASnakeCaseExample"))  # Output: this_is_a_snake_case_example
print(to_snake("STARTINGWithAllCapsDoesntWork")) # Output: starting_with_all_caps_doesnt_work # works :)
print(to_snake("startingWithLowerCaseDoesntWork")) # Output: starting_with_lower_case_doesnt_work # see disclaimer