import re


def check_dna(seq, code=re.compile(r'[^ACGTN.]').search):
    """Checks if a sequence consists of DNA-characters.
    
    Args:
        seq: Sequence, must be a string.
        code: Regex pattern.
    
    Returns:
        True: If the sequence is pure DNA.
        False: If the sequence if not (pure) DNA.

    """
    return not bool(code(seq))