def get_expressions():
    """Retrieve compiled pattern expressions.

    :returns: compiled regular expressions for supported filename formats
    :rtype: list
    """

    if len(_EXPRESSIONS) == len(FILENAME_PATTERNS):
        return _EXPRESSIONS

    for cpattern in FILENAME_PATTERNS:
        _EXPRESSIONS.append(re.compile(cpattern, re.VERBOSE))
    return _EXPRESSIONS