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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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