class_docstr: This class is responsible for the search of the appropriate files to use
  as input based on the constraints given in the config file.
code: def __str__(self):
      return self.to_string()
  
file_path: NiftyNet/niftynet/utilities/filename_matching.py
imports: from __future__ import absolute_import, print_function
  import os
  import re
  import six
  import niftynet.io.misc_io
  import tensorflow
label_desc: Use image node_id 7 for calling the KeywordsMatching obj's underlying member method code with example usage: obj.__str__() and returns: self