import re


def picard_friendly(umi):
    """
    Converts a UMI string into a format that Picard Tools can understand

    >>> picard_friendly("ACGT+ACGT")
    'ACGT-ACGT'
    >>> picard_friendly("ACGT_ACGT")
    'ACGT-ACGT'
    >>> picard_friendly("TTTAA")
    'TTTAA'
    """
    return re.sub("[^actgnACTGN]", "-", umi)