camelCase to snake_case conversion - Fixes #9726 (#9727)

* First commit

camel case to snake case conversion algorithm, including numbers

* code modified to not use regex

strings/camel_case_to_snake_case.py

@@ -0,0 +1,60 @@
+def camel_to_snake_case(input_str: str) -> str:
+    """
+    Transforms a camelCase (or PascalCase) string to snake_case
+
+    >>> camel_to_snake_case("someRandomString")
+    'some_random_string'
+
+    >>> camel_to_snake_case("SomeRandomStr#ng")
+    'some_random_str_ng'
+
+    >>> camel_to_snake_case("123someRandom123String123")
+    '123_some_random_123_string_123'
+
+    >>> camel_to_snake_case("123SomeRandom123String123")
+    '123_some_random_123_string_123'
+
+    >>> camel_to_snake_case(123)
+    Traceback (most recent call last):
+        ...
+    ValueError: Expected string as input, found <class 'int'>
+
+    """
+
+    # check for invalid input type
+    if not isinstance(input_str, str):
+        msg = f"Expected string as input, found {type(input_str)}"
+        raise ValueError(msg)
+
+    snake_str = ""
+
+    for index, char in enumerate(input_str):
+        if char.isupper():
+            snake_str += "_" + char.lower()
+
+        # if char is lowercase but proceeded by a digit:
+        elif input_str[index - 1].isdigit() and char.islower():
+            snake_str += "_" + char
+
+        # if char is a digit proceeded by a letter:
+        elif input_str[index - 1].isalpha() and char.isnumeric():
+            snake_str += "_" + char.lower()
+
+        # if char is not alphanumeric:
+        elif not char.isalnum():
+            snake_str += "_"
+
+        else:
+            snake_str += char
+
+    # remove leading underscore
+    if snake_str[0] == "_":
+        snake_str = snake_str[1:]
+
+    return snake_str
+
+
+if __name__ == "__main__":
+    from doctest import testmod
+
+    testmod()