build_match_and_apply_functions() is a function that builds other functions dynamically. It takes pattern, search and replace, then defines a matches_rule() function which calls re.search() with the pattern that was passed to the build_match_and_apply_functions() function, and the word that was passed to the matches_rule() function you’re building. Whoa.
re.sub() with the search and replace parameters that were passed to the build_match_and_apply_functions() function, and the word that was passed to the apply_rule() function you’re building. This technique of using the values of outside parameters within a dynamic function is called closures. You’re essentially defining constants within the apply function you’re building: it takes one parameter (word), but it then acts on that plus two other values (search and replace) which were set when you defined the apply function.
-build_match_and_apply_functions() function returns a tuple of two values: the two functions you just created. The constants you defined within those functions (pattern within the match_rule() function, and search and replace within the apply_rule() function) stay with those functions, even after you return from build_match_and_apply_functions(). That’s insanely cool.
+build_match_and_apply_functions() function returns a tuple of two values: the two functions you just created. The constants you defined within those functions (pattern within the matches_rule() function, and search and replace within the apply_rule() function) stay with those functions, even after you return from build_match_and_apply_functions(). That’s insanely cool.
If this is incredibly confusing (and it should be, this is weird stuff), it may become clearer when you see how to use it.