diff --git a/c-programming/strings/str_replace.c b/c-programming/strings/str_replace.c
index e545cbe..9a6c658 100644
--- a/c-programming/strings/str_replace.c
+++ b/c-programming/strings/str_replace.c
@@ -13,7 +13,8 @@ char* str_replace(
   const char* str,
   const char* substr,
   const char* replacement,
-  ssize_t max_count
+  size_t max_count,
+  bool direction
 ) {
 	/*
 	 * input arguments:
@@ -24,6 +25,15 @@ char* str_replace(
 	size_t substr_len = strlen(substr),
 	       replacement_len = strlen(replacement),
 	       count = 0;
+
+	if (direction == STR_REPLACE_DIR_BACKWARD) {
+		// TODO: implement replacing from the end
+		fprintf(stderr, "str_replace, STR_REPLACE_DIR_BACKWARD: Work in "
+		                "progress\n");
+		return str_replace(str, substr, replacement, max_count,
+		                   STR_REPLACE_DIR_FORWARD);
+	}
+
 	const char* p = str;
 
 	// count the number of occurrences of the substring
@@ -58,16 +68,59 @@ char* str_replace(
 }
 
 #ifdef TEST
-#include <stdio.h>
+# include <stdint.h>
+# include <stdio.h>
+
+uintmax_t tests_failed = 0;
+
+static void func_expect(
+  const char* str,
+  const char* substr,
+  const char* replacement,
+  size_t max_count,
+  bool direction,
+  const char* expected_output
+);
+
+static void func_expect(
+  const char* str,
+  const char* substr,
+  const char* replacement,
+  size_t max_count,
+  bool direction,
+  const char* expected_output
+) {
+	char* result = str_replace(str, substr, replacement, max_count, direction);
+	if (result == NULL) {
+		perror("str_replace");
+		exit(EXIT_FAILURE);
+	}
+
+	if (strcmp(result, expected_output)) {
+		puts("Failed!\n");
+		tests_failed++;
+		return;
+	}
+
+	puts("Passed\n");
+}
 
 int main(void) {
+	func_expect("Hello; peace! This is a peaceful test.",
+	            "; peace",
+	            ", universe",
+	            STR_REPLACE_ALL,
+	            STR_REPLACE_DIR_FORWARD,
+	            "Hello, universe! This is a peaceful test.");
 	const char* str = "Hello; peace! This is a peaceful test.",
 	          * substr = "peace",
 	          * replacement1 = "universe",
 	          * replacement2 = "_____";
 
-	char* result1 = str_replace(str, substr, replacement1, STR_REPLACE_ALL),
-	    * result2 = str_replace(str, substr, replacement2, 1);
+	char* result1 = str_replace(str, substr, replacement1, STR_REPLACE_ALL,
+	                            STR_REPLACE_DIR_FORWARD),
+	    * result2 = str_replace(str, substr, replacement2, 1,
+	                            STR_REPLACE_DIR_FORWARD);
 
 	puts(result1); free(result1);
 	puts(result2); free(result2);
diff --git a/c-programming/strings/str_replace.h b/c-programming/strings/str_replace.h
index 4cbc90f..55de70f 100644
--- a/c-programming/strings/str_replace.h
+++ b/c-programming/strings/str_replace.h
@@ -8,6 +8,7 @@
 #ifndef _STR_REPLACE_H
 #define _STR_REPLACE_H
 
+#include <stdbool.h>
 #include <stdlib.h>
 #include <string.h>