diff --git a/src/expansion/expansion.c b/src/expansion/expansion.c
index fe28dd0..dca13ec 100644
--- a/src/expansion/expansion.c
+++ b/src/expansion/expansion.c
@@ -1,3 +1,4 @@
+#define _POSIX_C_SOURCE 200809L
 #include <ctype.h>
 #include <stdbool.h>
 #include <stdio.h>
@@ -5,23 +6,113 @@
 #include <string.h>
 
 #include "../utils/ast/ast.h"
+#include "../utils/hash_map/hash_map.h"
+#include "../utils/string_utils/string_utils.h"
+#include "../utils/vars/vars.h"
 
-// static size_t var_len(char *start)
-// {
-//     char *iter = start;
-//     while (*iter != ' ' && *iter != 0)
-//         *iter++;
-//     return iter - start;
-// }
+static bool is_var_start_char(char c)
+{
+    return isalpha(c) || c == '_';
+}
 
-struct ast_command *expand(struct ast_command *command)
+static bool is_var_char(char c)
+{
+    return isalnum(c) || c == '_';
+}
+
+static bool is_special_var_char(char c)
+{
+    return c == '@' || c == '*' || c == '?' || c == '$' || isdigit(c)
+        || c == '#';
+}
+
+size_t parse_var_name(char *str, char **res)
+{
+    char *brace = NULL;
+    size_t i = 1; // skip the '$'
+
+    if (str[i] == '{' && str[i + 1] != 0 && str[i + 1] != '}')
+    {
+        if (is_special_var_char(str[i + 1]) && str[i + 2] == '}')
+        {
+            // Special variable like ${1}, ${?}
+            *res = strndup(str + i + 1, 1);
+            return 4; // length of ${X}
+        }
+
+        brace = str + i;
+        i++; // skip the '{'
+    }
+    else if (is_special_var_char(str[i]))
+    {
+        *res = strndup(str + i, 1);
+        return 2; // length of $X
+    }
+
+    if (!is_var_start_char(str[i]))
+    {
+        // Not a valid variable start
+        *res = NULL;
+        return 0;
+    }
+
+    while (1)
+    {
+        if (str[i] == '}' && *brace == '{')
+        {
+            *res = strndup(str + 2, i - 2);
+            return i + 1;
+        }
+        else if (!is_var_char(str[i]))
+        {
+            if (brace != NULL)
+            {
+                // Missing closing '}'
+                *res = NULL;
+                return 0;
+            }
+            break;
+        }
+        i++;
+    }
+
+    *res = strndup(str + 1, i - 1);
+    return i;
+}
+
+static bool expand_var(char **str, size_t pos, const struct hash_map *vars)
+{
+    char *var_name = NULL;
+    size_t r = parse_var_name(*str + pos, &var_name);
+    if (r > 0 && var_name != NULL)
+    {
+        char *value = get_var_or_env(vars, var_name);
+        if (value == NULL)
+            // Undefined variable: expand to empty string
+            value = "";
+
+        char *p = insert_into(*str, value, pos, r);
+        free(var_name);
+        if (p == NULL)
+        {
+            // error: insertion failed
+            return false;
+        }
+        *str = p;
+        return true;
+    }
+    return false;
+}
+
+struct ast_command *expand(struct ast_command *command,
+                           const struct hash_map *vars)
 {
     if (command == NULL)
         return NULL;
 
-    bool in_quotes = false;
     char *str;
     size_t len;
+    bool in_quotes;
     struct list *l = command->command;
 
     while (l != NULL)
@@ -30,31 +121,35 @@ struct ast_command *expand(struct ast_command *command)
         str = (char *)l->data;
         len = strlen(str);
 
-        for (size_t i = 0; str[i] != '\0'; i++)
+        for (size_t i = 0; str[i] != 0; i++)
         {
-            if (in_quotes)
-            {
-                // do nothing
-            }
-            else if (str[i] == '\'')
+            if (str[i] == '\'')
             {
+                // remove quote
                 in_quotes = !in_quotes;
-                memmove(&str[i], &str[i + 1], strlen(&str[i + 1]) + 1);
+                memmove(str + i, str + i + 1, strlen(str + i + 1) + 1);
+                i--;
+            }
+            else if (in_quotes)
+            {
+                continue; // do nothing
             }
             else if (str[i] == '$' && str[i + 1] != 0 && !isspace(str[i + 1]))
             {
-                // size_t len = var_len(str + i + 1);
-                // char *end = str + i + len + 1;
-                // char c = *end;
-                // *end = 0;
-                // printf("var: %s\n", str + i + 1);
-                // *end = c;
+                // variable expansion
+                bool r = expand_var(&str, i, vars);
+                if (r == false || str == NULL)
+                    return NULL;
+
+                i--; // -1 because loop will increment i
             }
         }
 
         if (in_quotes)
         {
             // error: quote not closed
+            fprintf(stderr, "Error: quote not closed in string: %s\n", str);
+            return NULL;
         }
 
         if (len != strlen(str))
@@ -63,6 +158,7 @@ struct ast_command *expand(struct ast_command *command)
             if (new_str == NULL)
             {
                 // error: realloc fail
+                return NULL;
             }
             l->data = new_str;
         }
@@ -71,17 +167,3 @@ struct ast_command *expand(struct ast_command *command)
     }
     return command;
 }
-
-// int main()
-// {
-//     printf("Expansion module test\n");
-//     struct ast_command ast_command;
-//     // char str[] = "echo Hello $?";
-//     char str[] = "echo Hello $AE86";
-//     ast_command.command = list_append(NULL, str);
-
-//     struct ast_command *command2 = expand(&ast_command);
-//     printf("command2: %s\n", (char *)command2->command->data);
-
-//     return 0;
-// }
diff --git a/src/expansion/expansion.h b/src/expansion/expansion.h
index b10b198..7211ae3 100644
--- a/src/expansion/expansion.h
+++ b/src/expansion/expansion.h
@@ -1,4 +1,26 @@
 #ifndef EXPANSION_H
 #define EXPANSION_H
 
+#include <stddef.h>
+
+#include "../utils/hash_map/hash_map.h"
+
+/**
+ * Parse a variable from a string starting with '$'.
+ * @param str The input string starting with '$'. It must start with '$'.
+ * @param res Pointer to a char pointer that will be set to the extracted
+ * variable name.
+ * @return The number of characters processed in the input string.
+ */
+size_t parse_var_name(char *str, char **res);
+
+/**
+ * Expand variables in an AST command using the provided variable map.
+ * @param command The AST command to expand.
+ * @param vars The hash map containing variables.
+ * @return A new AST command with variables expanded, or NULL on error.
+ */
+struct ast_command *expand(struct ast_command *command,
+                           const struct hash_map *vars);
+
 #endif /* ! EXPANSION_H */
diff --git a/src/utils/hash_map/hash_map.c b/src/utils/hash_map/hash_map.c
index dec698c..46ac9d1 100644
--- a/src/utils/hash_map/hash_map.c
+++ b/src/utils/hash_map/hash_map.c
@@ -28,6 +28,14 @@ static size_t hash(const char *key)
     return hash;
 }
 
+static void destroy_pair_list(struct pair_list **p)
+{
+    free((char *)(*p)->key);
+    free((*p)->value);
+    free((*p));
+    *p = NULL;
+}
+
 struct hash_map *hash_map_init(size_t size)
 {
     struct hash_map *p = malloc(sizeof(struct hash_map));
@@ -102,7 +110,7 @@ void hash_map_free(struct hash_map *hash_map)
             {
                 prev = l;
                 l = l->next;
-                free(prev);
+                destroy_pair_list(&prev);
             }
         }
         free(hash_map->data);
@@ -163,7 +171,7 @@ bool hash_map_remove(struct hash_map *hash_map, const char *key)
                 p->next = l->next;
             else
                 hash_map->data[i] = l->next;
-            free(l);
+            destroy_pair_list(&l);
             return true;
         }
         p = l;
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/src/utils/vars/vars.c b/src/utils/vars/vars.c
index 70ab328..985e085 100644
--- a/src/utils/vars/vars.c
+++ b/src/utils/vars/vars.c
@@ -2,7 +2,7 @@
 #include "vars.h"
 
 #include <stdbool.h>
-#include <stddef.h>
+#include <stdlib.h>
 #include <string.h>
 
 #include "../hash_map/hash_map.h"
@@ -19,6 +19,14 @@ char *get_var(const struct hash_map *vars, const char *key)
     return (char *)hash_map_get(vars, key);
 }
 
+char *get_var_or_env(const struct hash_map *vars, const char *key)
+{
+    char *value = (char *)hash_map_get(vars, key);
+    if (value == NULL)
+        value = getenv(key);
+    return value;
+}
+
 bool set_var(struct hash_map *vars, const char *key, const char *value)
 {
     if (key == NULL || value == NULL)
diff --git a/src/utils/vars/vars.h b/src/utils/vars/vars.h
index 97db704..de1c4c6 100644
--- a/src/utils/vars/vars.h
+++ b/src/utils/vars/vars.h
@@ -15,6 +15,12 @@ struct hash_map *vars_init(void);
  */
 char *get_var(const struct hash_map *vars, const char *key);
 
+/**
+ * Get the value of a variable, from the environment if not found in vars,
+ * NULL if not found in either.
+ */
+char *get_var_or_env(const struct hash_map *vars, const char *key);
+
 /**
  * Set the value of a variable. Key and value ownership are transferred to
  * the hash_map and need to be on the heap. Returns true on success, false on
diff --git a/tests/unit/expansion/expand.c b/tests/unit/expansion/expand.c
new file mode 100644
index 0000000..1e25d7c
--- /dev/null
+++ b/tests/unit/expansion/expand.c
@@ -0,0 +1,220 @@
+#define _POSIX_C_SOURCE 200809L
+#include <criterion/criterion.h>
+#include <criterion/new/assert.h>
+#include <criterion/redirect.h>
+#include <stdlib.h>
+
+#include "../../../src/expansion/expansion.h"
+#include "../../../src/utils/ast/ast.h"
+#include "../../../src/utils/hash_map/hash_map.h"
+#include "../../../src/utils/vars/vars.h"
+
+TestSuite(expand);
+
+Test(expand, no_expansion)
+{
+    char str[] = "echo something";
+    char *str_heap = strdup(str);
+    struct list *list = list_append(NULL, str_heap);
+    struct ast *ast = ast_create_command(list);
+    struct ast_command *ast_command = ast_get_command(ast);
+
+    struct ast_command *command2 = expand(ast_command, NULL);
+    cr_assert_not_null(command2, "Expansion returned NULL");
+    cr_assert_str_eq((char *)command2->command->data, "echo something",
+                     "String without variables should remain unchanged");
+    ast_free(&ast);
+}
+
+Test(expand, single_quotes_no_expansion)
+{
+    char str[] = "echo '$VAR'";
+    char *str_heap = strdup(str);
+    struct list *list = list_append(NULL, str_heap);
+    struct ast *ast = ast_create_command(list);
+    struct ast_command *ast_command = ast_get_command(ast);
+
+    struct hash_map *vars = vars_init();
+    set_var_copy(vars, "VAR", "expanded");
+
+    struct ast_command *command2 = expand(ast_command, vars);
+    cr_assert_not_null(command2, "Expansion returned NULL");
+    cr_assert_str_eq((char *)command2->command->data, "echo $VAR",
+                     "Variable should not expand inside single quotes");
+    ast_free(&ast);
+    hash_map_free(vars);
+}
+
+Test(expand, single_dollar)
+{
+    char str[] = "echo $ sign";
+    char *str_heap = strdup(str);
+    struct list *list = list_append(NULL, str_heap);
+    struct ast *ast = ast_create_command(list);
+    struct ast_command *ast_command = ast_get_command(ast);
+
+    struct hash_map *vars = vars_init();
+    set_var_copy(vars, "VAR", "expanded");
+
+    struct ast_command *command2 = expand(ast_command, vars);
+    cr_assert_not_null(command2, "Expansion returned NULL");
+    cr_assert_str_eq((char *)command2->command->data, "echo $ sign",
+                     "Variable should not expand inside single quotes");
+    ast_free(&ast);
+    hash_map_free(vars);
+}
+
+Test(expand, empty_braces_no_expansion)
+{
+    char str[] = "echo ${}";
+    char *str_heap = strdup(str);
+    struct list *list = list_append(NULL, str_heap);
+    struct ast *ast = ast_create_command(list);
+    struct ast_command *ast_command = ast_get_command(ast);
+
+    struct hash_map *vars = vars_init();
+    set_var_copy(vars, "VAR", "expanded");
+
+    struct ast_command *command2 = expand(ast_command, vars);
+    cr_assert_null(command2, "Expansion should fail on empty braces");
+    ast_free(&ast);
+    hash_map_free(vars);
+}
+
+Test(expand, basic_expansion)
+{
+    char str[] = "echo $VAR";
+    char *str_heap = strdup(str);
+    struct list *list = list_append(NULL, str_heap);
+    struct ast *ast = ast_create_command(list);
+    struct ast_command *ast_command = ast_get_command(ast);
+
+    struct hash_map *vars = vars_init();
+    set_var_copy(vars, "VAR", "expanded");
+
+    struct ast_command *command2 = expand(ast_command, vars);
+    cr_assert_not_null(command2, "Expansion returned NULL");
+    cr_assert_str_eq((char *)command2->command->data, "echo expanded",
+                     "Variable should expand correctly");
+    ast_free(&ast);
+    hash_map_free(vars);
+}
+
+Test(expand, multiple_expansion)
+{
+    char str[] = "echo $VAR1 $VAR2 ${VAR3}";
+    char *str_heap = strdup(str);
+    struct list *list = list_append(NULL, str_heap);
+    struct ast *ast = ast_create_command(list);
+    struct ast_command *ast_command = ast_get_command(ast);
+
+    struct hash_map *vars = vars_init();
+    set_var_copy(vars, "VAR1", "expanded");
+    set_var_copy(vars, "VAR2", "values");
+    set_var_copy(vars, "VAR3", "here");
+
+    struct ast_command *command2 = expand(ast_command, vars);
+    cr_assert_not_null(command2, "Expansion returned NULL");
+    cr_assert_str_eq((char *)command2->command->data,
+                     "echo expanded values here",
+                     "Multiple variables should expand correctly");
+    ast_free(&ast);
+    hash_map_free(vars);
+}
+
+Test(expand, env_variable)
+{
+    char str[] = "echo $MY_ENV_VAR";
+    char *str_heap = strdup(str);
+    struct list *list = list_append(NULL, str_heap);
+    struct ast *ast = ast_create_command(list);
+    struct ast_command *ast_command = ast_get_command(ast);
+
+    setenv("MY_ENV_VAR", "environment", 0);
+
+    struct ast_command *command2 = expand(ast_command, NULL);
+    cr_assert_not_null(command2, "Expansion returned NULL");
+    cr_assert_str_eq((char *)command2->command->data, "echo environment",
+                     "Environment variable should expand correctly");
+    ast_free(&ast);
+}
+
+Test(expand, undefined_variable)
+{
+    char str[] = "echo $UNDEFINED";
+    char *str_heap = strdup(str);
+    struct list *list = list_append(NULL, str_heap);
+    struct ast *ast = ast_create_command(list);
+    struct ast_command *ast_command = ast_get_command(ast);
+
+    struct hash_map *vars = vars_init();
+
+    struct ast_command *command2 = expand(ast_command, vars);
+    cr_assert_not_null(command2, "Expansion returned NULL");
+    cr_assert_str_eq((char *)command2->command->data, "echo ",
+                     "Undefined variable should expand to empty string");
+    ast_free(&ast);
+    hash_map_free(vars);
+}
+
+Test(expand, nested_expansion)
+{
+    char str[] = "echo $B";
+    char *str_heap = strdup(str);
+    struct list *list = list_append(NULL, str_heap);
+    struct ast *ast = ast_create_command(list);
+    struct ast_command *ast_command = ast_get_command(ast);
+
+    struct hash_map *vars = vars_init();
+    set_var_copy(vars, "A", "expanded");
+    set_var_copy(vars, "B", "$A");
+
+    struct ast_command *command2 = expand(ast_command, vars);
+    cr_assert_not_null(command2, "Expansion returned NULL");
+    cr_assert_str_eq((char *)command2->command->data, "echo expanded",
+                     "Nested variable should expand correctly");
+    ast_free(&ast);
+    hash_map_free(vars);
+}
+
+Test(expand, mixed_quotes_expansion)
+{
+    char str[] = "echo \"$VAR1 and '$VAR2'\"";
+    char *str_heap = strdup(str);
+    struct list *list = list_append(NULL, str_heap);
+    struct ast *ast = ast_create_command(list);
+    struct ast_command *ast_command = ast_get_command(ast);
+
+    struct hash_map *vars = vars_init();
+    set_var_copy(vars, "VAR1", "expanded");
+    set_var_copy(vars, "VAR2", "not_expanded");
+
+    struct ast_command *command2 = expand(ast_command, vars);
+    cr_assert_not_null(command2, "Expansion returned NULL");
+    cr_assert_str_eq((char *)command2->command->data,
+                     "echo \"expanded and $VAR2\"",
+                     "Variable in double quotes should expand, while variable "
+                     "in single quotes should not");
+    ast_free(&ast);
+    hash_map_free(vars);
+}
+
+Test(expand, adjacent_variables)
+{
+    char str[] = "echo $VAR1$VAR2";
+    char *str_heap = strdup(str);
+    struct list *list = list_append(NULL, str_heap);
+    struct ast *ast = ast_create_command(list);
+    struct ast_command *ast_command = ast_get_command(ast);
+
+    struct hash_map *vars = vars_init();
+    set_var_copy(vars, "VAR1", "hello");
+    set_var_copy(vars, "VAR2", "world");
+
+    struct ast_command *command2 = expand(ast_command, vars);
+    cr_assert_not_null(command2, "Expansion returned NULL");
+    cr_assert_str_eq((char *)command2->command->data, "echo helloworld",
+                     "Adjacent variables should expand correctly");
+    ast_free(&ast);
+    hash_map_free(vars);
+}
diff --git a/tests/unit/expansion/parse_var.c b/tests/unit/expansion/parse_var.c
new file mode 100644
index 0000000..27a4b94
--- /dev/null
+++ b/tests/unit/expansion/parse_var.c
@@ -0,0 +1,144 @@
+#include <criterion/criterion.h>
+#include <criterion/new/assert.h>
+#include <criterion/redirect.h>
+
+#include "../../../src/expansion/expansion.h"
+
+TestSuite(parse_var_name);
+
+Test(parse_var_name, basic_variable)
+{
+    char *input = "$MY_VAR";
+    char *extracted_var = NULL;
+    size_t r = parse_var_name(input, &extracted_var);
+
+    cr_expect(r == 7);
+    cr_expect_str_eq(extracted_var, "MY_VAR");
+    free(extracted_var);
+}
+
+Test(parse_var_name, multi_basic_variable)
+{
+    char *input = "$MY$VAR";
+    char *extracted_var = NULL;
+    size_t r = parse_var_name(input, &extracted_var);
+
+    cr_expect(r == 3);
+    cr_expect_str_eq(extracted_var, "MY");
+    free(extracted_var);
+
+    input += r;
+    r = parse_var_name(input, &extracted_var);
+
+    cr_expect(r == 4);
+    cr_expect_str_eq(extracted_var, "VAR");
+    free(extracted_var);
+}
+
+Test(parse_var_name, variable_with_braces)
+{
+    char *input = "${MY_VAR}";
+    char *extracted_var = NULL;
+    size_t r = parse_var_name(input, &extracted_var);
+
+    cr_expect(r == 9);
+    cr_expect_str_eq(extracted_var, "MY_VAR");
+    free(extracted_var);
+}
+
+Test(parse_var_name, special_variable)
+{
+    char *input = "$1";
+    char *extracted_var = NULL;
+    size_t r = parse_var_name(input, &extracted_var);
+
+    cr_expect(r == 2);
+    cr_expect_str_eq(extracted_var, "1");
+    free(extracted_var);
+}
+
+Test(parse_var_name, special_variable_with_braces)
+{
+    char *input = "${1}";
+    char *extracted_var = NULL;
+    size_t r = parse_var_name(input, &extracted_var);
+
+    cr_expect(r == 4);
+    cr_expect_str_eq(extracted_var, "1");
+    free(extracted_var);
+}
+
+Test(parse_var_name, incomplete_braces)
+{
+    char *input = "${MY_VAR";
+    char *extracted_var = NULL;
+    size_t r = parse_var_name(input, &extracted_var);
+
+    cr_expect(r == 0);
+    cr_expect(extracted_var == NULL);
+}
+
+Test(parse_var_name, empty_braces)
+{
+    char *input = "${}";
+    char *extracted_var = NULL;
+    size_t r = parse_var_name(input, &extracted_var);
+
+    cr_expect(r == 0);
+    cr_expect(extracted_var == NULL);
+}
+
+Test(parse_var_name, dollar_sign_only)
+{
+    char *input = "$";
+    char *extracted_var = NULL;
+    size_t r = parse_var_name(input, &extracted_var);
+
+    cr_expect(r == 0);
+    cr_expect(extracted_var == NULL);
+}
+
+Test(parse_var_name, variable_followed_by_dollar)
+{
+    char *input = "$MY$VAR$";
+    char *extracted_var = NULL;
+    size_t r = parse_var_name(input, &extracted_var);
+
+    cr_expect(r == 3);
+    cr_expect_str_eq(extracted_var, "MY");
+    free(extracted_var);
+
+    input += r;
+    r = parse_var_name(input, &extracted_var);
+
+    cr_expect(r == 4);
+    cr_expect_str_eq(extracted_var, "VAR");
+    free(extracted_var);
+
+    input += r;
+    r = parse_var_name(input, &extracted_var);
+
+    cr_expect(r == 0);
+    cr_expect(extracted_var == NULL);
+}
+
+Test(parse_var_name, special_variable_followed_by_text)
+{
+    char *input = "$1VAR";
+    char *extracted_var = NULL;
+    size_t r = parse_var_name(input, &extracted_var);
+
+    cr_expect(r == 2);
+    cr_expect_str_eq(extracted_var, "1");
+    free(extracted_var);
+}
+
+Test(parse_var_name, bad_variable_with_braces)
+{
+    char *input = "${1VAR}";
+    char *extracted_var = NULL;
+    size_t r = parse_var_name(input, &extracted_var);
+
+    cr_expect(r == 0);
+    cr_expect(extracted_var == NULL);
+}
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);
+}