def refine(self, filelist, regex, filterdir='out', **kwargs):
        ''' Returns a list of files filterd by a regular expression

        Parameters
        ----------
        filelist : list
            A list of files to filter on.

        regex : str
            The regular expression string to filter your list

        filterdir: {'in', 'out'}
            Indicates the filter to be inclusive or exclusive
            'out' removes the items satisfying the regular expression
            'in' keeps the items satisfying the regular expression

        Returns
        -------
        refine : list
            A file list refined by an input regular expression.

        '''
        assert filelist, 'Must provide a list of filenames to refine on'
        assert regex, 'Must provide a regular expression to refine the file list'
        r = re.compile(regex)

        # icheck filter direction; default is out
        assert filterdir in ['in', 'out'], 'Filter direction must be either "in" or "out"'
        if filterdir == 'out':
            subset = list(filter(lambda i: r.search(i), filelist))
        elif filterdir == 'in':
            subset = list(filter(lambda i: not r.search(i), filelist))
        return subset
Returns a list of files filterd by a regular expression

        Parameters
        ----------
        filelist : list
            A list of files to filter on.

        regex : str
            The regular expression string to filter your list

        filterdir: {'in', 'out'}
            Indicates the filter to be inclusive or exclusive
            'out' removes the items satisfying the regular expression
            'in' keeps the items satisfying the regular expression

        Returns
        -------
        refine : list
            A file list refined by an input regular expression.