Rename variable from `REGEX' to `FLAG_FILTER' massively: 
    
        - The regular expression value stored in this variable is
          initiated in cli.sh and later redefined, at need, by means of
          --filter='regex' command-line argument. Where `--filter' is the
          name of the flag, and `regex' the regular expression itself.
          Quotation is useful to avoid problems with space characters
          inside the regular expression.