Scans the student files for text patterns.

        Args:
            regex (str):       Regular expression used for scanning inside the files.

        Returns:
            tuple:     Names of the matching files in the working directory.

def grep(self, regex):
        """Scans the student files for text patterns.

        Args:
            regex (str):       Regular expression used for scanning inside the files.

        Returns:
            tuple:     Names of the matching files in the working directory.
        """
        matches = []
        logger.debug("Searching student files for '{0}'".format(regex))
        for fname in self.student_files:
            if os.path.isfile(self.working_dir + fname):
                for line in open(self.working_dir + fname, 'br'):
                    if re.search(regex.encode(), line):
                        logger.debug("{0} contains '{1}'".format(fname, regex))
                        matches.append(fname)
        return matches