Implement matching strings + finding groups.

They're done in the same step.

result = _regex_find_all(root, &clear_ll, -1);
ll_clear(&clear_ll);

Is all that's necessary to reset all nodes, since the only value
that changes during the match is list_id, and that gets set when the
nodes are found.

include/libregex.h

@@ -300,6 +300,7 @@ void regex_node_clear(regex_node* node);
 /**
  * Frees an entire NFA, starting from the given root node.
  * @param root the root, or starting node, of the NFA.
+ * @return the result of the operation: LIBREGEX_SUCCESS if all goes well, or an error code.
  */
 libregex_result regex_free(regex_node* root);
 /**
@@ -317,5 +318,11 @@ libregex_result regex_build(regex_node** root, char* expression);
  * @return the result of the operation: LIBREGEX_SUCCESS if all goes well, or an error code.
  */
 libregex_result regex_match_string(regex_node* root, char* string, regex_result* result);
+/**
+ * Frees data used by regex_match_string in a regex_result struct.
+ * The actual struct is not freed.
+ * @param result the result struct
+ */
+void regex_result_free(regex_result* result);
 
 #endif