import re


def sanitize_file_path(file):
    """
    Replaces backslashes with forward slashes and removes leading / or drive:/.

    :param file: Relative or absolute filepath
    :return: Relative or absolute filepath with leading / or drive:/ removed
    """
    # Sanitize all backslashes (\) with forward slashes (/)
    file_key = file.replace('\\', '/').replace('//', '/')
    # If Mac/Linux full path
    if re.search(r'^/.+$', file):
        file_key = file.split('/', 1)[1]
    # If Windows full path
    elif re.search(r'^\D:/.+$', file_key):
        file_key = file_key.split(':/', 1)[1]
    return file_key