diff --git a/interesting_tests.md b/interesting_tests.md
new file mode 100644
index 0000000..22358b9
--- /dev/null
+++ b/interesting_tests.md
@@ -0,0 +1,28 @@
+jguelen@c2r12p1 ~()% bash
+jguelen@c2r12p1:~$ export test=flagada$PATH
+jguelen@c2r12p1:~$ echo $test
+flagada/home/jguelen/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin:/usr/local/go/bin
+jguelen@c2r12p1:~$ export test="flagada"$PATH
+jguelen@c2r12p1:~$ echo $test
+flagada/home/jguelen/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin:/usr/local/go/bin
+jguelen@c2r12p1:~$ export test=flagada"$PATHhihi"
+jguelen@c2r12p1:~$ echo $test
+flagada
+jguelen@c2r12p1:~$ export test=flagada"$PATH hihi"
+jguelen@c2r12p1:~$ echo $test
+flagada/home/jguelen/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin:/usr/local/go/bin hihi
+jguelen@c2r12p1:~$ export test=flagada"$PATH"hihi
+jguelen@c2r12p1:~$ echo $test
+flagada/home/jguelen/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin:/usr/local/go/binhihi
+jguelen@c2r12p1:~$ export test=flagada"$'PA'TH"hihi
+jguelen@c2r12p1:~$ echo $test
+flagada$'PA'THhihi
+jguelen@c2r12p1:~$ export test=flagada$'P'ATH
+jguelen@c2r12p1:~$ echo $test
+flagadaPATH
+jguelen@c2r12p1:~$ export test=flagada$P'A'TH hihi
+jguelen@c2r12p1:~$ echo $test
+flagadaATH
+jguelen@c2r12p1:~/Common_Core/minishell$ echo "tests$"PATH""
+tests$PATH
+jguelen@c2r12p1:~$
diff --git a/src/subst/replace_substr.c b/src/subst/replace_substr.c
new file mode 100644
index 0000000..dc8c7c4
--- /dev/null
+++ b/src/subst/replace_substr.c
@@ -0,0 +1,131 @@
+/* ************************************************************************** */
+/*                                                                            */
+/*                                                        :::      ::::::::   */
+/*   replace_substr.c                                   :+:      :+:    :+:   */
+/*                                                    +:+ +:+         +:+     */
+/*   By: jguelen <marvin@42.fr>                     +#+  +:+       +#+        */
+/*                                                +#+#+#+#+#+   +#+           */
+/*   Created: 2025/02/25 13:02:59 by jguelen           #+#    #+#             */
+/*   Updated: 2025/02/25 18:50:00 by jguelen          ###   ########.fr       */
+/*                                                                            */
+/* ************************************************************************** */
+
+#include "replace_substr.h"
+
+/*
+** @RETURN Returns a new C-compliant malloc-allocated string corresponding to
+** text where the substring starting in text at index index_start and ending
+** at index_end is replaced in totality by the C-compliant string replacement
+** excluding its NULL-byte.
+** Returns NULL if an allocation error occurred.
+*/
+char	*replace_in_str(const char *text, size_t index_start, size_t index_end,
+			char *replacement)
+{
+	char	*new;
+	size_t	len_text;
+	size_t	len_rep;
+	size_t	new_size;
+
+	len_text = ft_strlen(text);
+	len_rep = ft_strlen(replacement);
+	new_size = len_text + index_start - index_end + len_rep;
+	if (index_end >= index_start)
+	{
+		new = malloc(new_size * sizeof(char));
+		if (!new)
+			return (NULL);
+		ft_memmove(new, text, index_start);
+		ft_memmove(new + index_start, replacement, len_rep);
+		ft_memmove(new + index_start + len_rep, text + index_end + 1,
+			len_text - index_end - 1);
+		new[new_size - 1] = '\0';
+		return (new);
+	}
+	return (ft_strdup(text));
+}
+
+/*
+** word is presumed not to be NULL
+*/
+int	*better_prefix_array(char *word, size_t word_len)
+{
+	int	i;
+	int	j;
+	int	*better_prefix;
+
+	i = 0;
+	better_prefix = ft_calloc(word_len + 1, sizeof(int));
+	if (!better_prefix)
+		return (NULL);
+	better_prefix[0] = -1;
+	j = 0;
+	while (++j < m)
+	{
+		if (word[i] == word[j])
+			better_prefix[j] = better_prefix[i];
+		else
+		{
+			better_prefix[j] = i;
+			i = better_prefix[i];
+			while (i >= 0 && word[j] != word[i])
+				i = better_prefix[i];
+		}
+		i++;
+	}
+	better_prefix[word_len] = i;
+	return (better_prefix);
+}
+
+/*
+** Return the index at which the word needle begins in haystack if found,
+** -1 if needle is not to be found in haystack.
+*/
+ssize_t	find_word_index(const char *haystack, char *needle)
+{
+	int		i;
+	int		j;
+	size_t	len_n;
+	int		*better_prefix;
+
+	i = 0;
+	j = 0;
+	len_n = ft_strlen(needle);
+	better_prefix = better_prefix_array(needle, len_n);
+	while (haystack[j])
+	{
+		while (i >= 0 && needle[i] != haystack[j])
+			i = better_prefix[i];
+		i++;
+		if (i == len_n)
+			return (free(better_prefix), j);
+		i = better_prefix[i];
+		j++;
+	}
+	free(better_prefix);
+	return (-1);
+}
+
+/*
+** @RETURN Returns a new C-compliant malloc-allocated string corresponding to
+** the C-compliant string text where the first occurrence of the C-compliant
+** string to_replace (NULL-byte excluded) found in text is replaced by the
+** C-compliant string replacement.
+** Returns NULL if an allocation error occurred.
+*/
+char	*replace_substr(const char *text, char *to_replace, char *replacement)
+{
+	char	*new;
+	size_t	start_index;
+	size_t	len_torep;
+
+	len_torep = ft_strlen(to_replace);
+	if (len_torep)
+	{
+		start_index = find_word_index(text, to_replace);
+		if (start_index != -1)
+			return (replace_in_str(text, index_start,
+					(size_t)start_index + len_torep - 1, replacement));
+	}
+	return (ft_strdup(text));
+}
diff --git a/src/subst/replace_substr.h b/src/subst/replace_substr.h
new file mode 100644
index 0000000..ee704e7
--- /dev/null
+++ b/src/subst/replace_substr.h
@@ -0,0 +1,17 @@
+/* ************************************************************************** */
+/*                                                                            */
+/*                                                        :::      ::::::::   */
+/*   replace_substr.h                                   :+:      :+:    :+:   */
+/*                                                    +:+ +:+         +:+     */
+/*   By: jguelen <jguelen@student.42.fr>            +#+  +:+       +#+        */
+/*                                                +#+#+#+#+#+   +#+           */
+/*   Created: 2025/02/25 12:48:39 by jguelen           #+#    #+#             */
+/*   Updated: 2025/02/25 13:03:51 by jguelen          ###   ########.fr       */
+/*                                                                            */
+/* ************************************************************************** */
+
+#include "libft.h"
+
+char	*replace_in_str(char *text, size_t index_start, size_t index_end,
+			char *replacement);
+char	*replace_substr(char *text, char *to_replace, char *replacement);