Options are passed to open/4 and phrase_from_stream/3. The latter notable allows for as(chars) to call a DCG compiled for dealing with character lists rather than codes.

Below is an example that counts the number of times a string appears in a file. The library dcg/basics provides string//1 matching an arbitrary string and remainder//1 which matches the remainder of the input without parsing.

:- use_module(library(dcg/basics)).

file_contains(File, Pattern) :-
        phrase_from_file(match(Pattern), File).

match(Pattern) -->
        string(_),
        string(Pattern),
        remainder(_).

match_count(File, Pattern, Count) :-
        aggregate_all(count, file_contains(File, Pattern), Count).

This can be called as (note that the pattern must be a string (code list)):

?- match_count('pure_input.pl', `file`, Count).