diff --git a/src/utils/string_utils/string_utils.c b/src/utils/string_utils/string_utils.c
index 8a8176a..e5b7040 100644
--- a/src/utils/string_utils/string_utils.c
+++ b/src/utils/string_utils/string_utils.c
@@ -1,7 +1,8 @@
 #include "string_utils.h"
 
 #include <ctype.h>
-#include <stddef.h>
+#include <stdlib.h>
+#include <string.h>
 
 char *trim_blank_left(char *str)
 {
@@ -13,3 +14,31 @@ char *trim_blank_left(char *str)
 
     return str;
 }
+
+char *insert_into(char *dest, const char *src, size_t pos, size_t len)
+{
+    size_t res_len = strlen(dest);
+    size_t prefix_len = pos;
+    size_t suffix_len = res_len - (pos + len);
+    size_t src_len = strlen(src);
+    size_t new_len = prefix_len + src_len + suffix_len;
+
+    if (dest == NULL || src == NULL || pos + len > res_len)
+        return NULL;
+
+    if (res_len < new_len)
+    {
+        char *p = realloc(dest, new_len + 1);
+        if (p == NULL)
+            return NULL; // allocation failure
+        dest = p;
+    }
+
+    memmove(dest + pos + src_len, dest + pos + len, suffix_len);
+    memcpy(dest + pos, src, src_len);
+    dest[new_len] = 0;
+
+    if (res_len > new_len)
+        return realloc(dest, new_len + 1);
+    return dest;
+}
diff --git a/src/utils/string_utils/string_utils.h b/src/utils/string_utils/string_utils.h
index 496c1d5..e411f0e 100644
--- a/src/utils/string_utils/string_utils.h
+++ b/src/utils/string_utils/string_utils.h
@@ -12,4 +12,10 @@
  */
 char *trim_blank_left(char *str);
 
+/**
+ * Inserts a substring into a destination string at a specified position,
+ * replacing a specified length of characters.
+ */
+char *insert_into(char *dest, const char *src, size_t pos, size_t len);
+
 #endif /* STRING_UTILS_H */
diff --git a/tests/unit/utils/insert_into.c b/tests/unit/utils/insert_into.c
new file mode 100644
index 0000000..0bcc833
--- /dev/null
+++ b/tests/unit/utils/insert_into.c
@@ -0,0 +1,85 @@
+#define _POSIX_C_SOURCE 200809L
+#include <criterion/criterion.h>
+#include <criterion/new/assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "../../../src/utils/string_utils/string_utils.h"
+
+TestSuite(insert_into);
+
+Test(insert_into, basic)
+{
+    char *dest = strdup("The <WORD> is nice.");
+    const char *src = "weather";
+    size_t pos = 4;
+
+    char *result = insert_into(dest, src, pos, 6);
+
+    cr_expect(result != NULL);
+    cr_expect(eq(str, result, "The weather is nice."));
+
+    if (result)
+        free(result);
+}
+
+Test(insert_into, begin)
+{
+    char *dest = strdup("Hello World!");
+    const char *src = "Hi";
+    size_t pos = 0;
+
+    char *result = insert_into(dest, src, pos, 5);
+
+    cr_expect(result != NULL);
+    cr_expect(eq(str, result, "Hi World!"));
+
+    if (result)
+        free(result);
+}
+
+Test(insert_into, end)
+{
+    char *dest = strdup("The number is 1024");
+    const char *src = "2048";
+    size_t pos = 14;
+
+    char *result = insert_into(dest, src, pos, 4);
+
+    cr_expect(result != NULL);
+    cr_expect(eq(str, result, "The number is 2048"));
+
+    if (result)
+        free(result);
+}
+
+Test(insert_into, big)
+{
+    char *dest = strdup("I could insert [VAR] here.");
+    const char *src = "a very very long string";
+    size_t pos = 15;
+
+    char *result = insert_into(dest, src, pos, 5);
+
+    cr_expect(result != NULL);
+    cr_expect(eq(str, result, "I could insert a very very long string here."));
+
+    if (result)
+        free(result);
+}
+
+Test(insert_into, small)
+{
+    char *dest = strdup("I could insert [VARNAME_IS_SO_LONG] string here.");
+    const char *src = "a short";
+    size_t pos = 15;
+
+    char *result = insert_into(dest, src, pos, 20);
+
+    cr_expect(result != NULL);
+    cr_expect(eq(str, result, "I could insert a short string here."));
+
+    if (result)
+        free(result);
+}