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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 construct_transformed_regex(annotations):
 """ construct a regex that matches possible fields in a transformed file annotations is a set of which keys in BARCODEINFO are present in the file """

    re_string = '.*'
    if "cellular" in annotations:
        re_string += ":CELL_(?P<CB>.*)"
    if "molecular" in annotations:
        re_string += ":UMI_(?P<MB>\w*)"
    if "sample" in annotations:
        re_string += ":SAMPLE_(?P<SB>\w*)"
    if re_string == ".*":
        logger.error("No annotation present on this file, aborting.")
        sys.exit(1)
    return re_string