import re


def is_label(test_string):
    """
    Test if a string is a valid label.

    Args:
        test_string (str): The string to test
    Returns:
        bool: True if the string is a valid label, false otherwise.
    """
    match = re.match(r"@[a-zA-Z_]+\w*$", test_string)
    if match:
        return True
    else:
        return False