A Python function to sanitize input strings by replacing non-alphanumeric characters with underscores, stripping leading/trailing underscores or spaces, and prepending an underscore if the string starts with a digit.

sanitize_string.py

import re

def sanitize_string(input_string, prepend_char='_'):
    """
    Replaces any character not in [a-zA-Z0-9_] with an underscore, strips leading and trailing underscores or spaces,
    and prepends a character if the first character is a digit.

    Args:
        input_string (str): The string to be sanitized.
        prepend_char (str): The character to prepend if the first character is a digit. Defaults to '_'.

    Returns:
        str: The sanitized, stripped, and possibly prepended string.

    Examples:
    >>> sanitize_string(" 1Example!String with various+chars&to#sanitize. ")
    '_1Example_String_with_various_chars_to_sanitize'
    >>> sanitize_string("Example with spaces and_underscores.")
    'Example_with_spaces_and_underscores'
    >>> sanitize_string("123numbers start")
    '_123numbers_start'
    >>> sanitize_string("no$pecial*Characters!")
    'no_pecial_Characters'
    >>> sanitize_string("__Already__Clean__")
    'Already__Clean'
    """
    # Replace non-standard characters and strip
    sanitized = re.sub(r'[^a-zA-Z0-9_]', '_', input_string).strip('_ ')
    # Prepend if the first character is a digit
    if sanitized and sanitized[0].isdigit():
        sanitized = prepend_char + sanitized
    return sanitized

if __name__ == "__main__":
    import doctest
    doctest.testmod()