diff --git a/include/common/regex.h b/include/common/regex.h
index 9080bda44..59730cac9 100644
--- a/include/common/regex.h
+++ b/include/common/regex.h
@@ -83,11 +83,20 @@ const char *check_replace_string(const char *str);
 const char *chain_regex(struct hdr_exp **head, const regex_t *preg,
 			int action, const char *replace, void *cond);
 
-static inline int regex_exec(const regex *preg, const char *subject, int length) {
+/* Note that <subject> MUST be at least <length+1> characters long and must
+ * be writable because the function will temporarily force a zero past the
+ * last character.
+ */
+static inline int regex_exec(const regex *preg, char *subject, int length) {
 #ifdef USE_PCRE_JIT
 	return pcre_exec(preg->reg, preg->extra, subject, length, 0, 0, NULL, 0);
 #else
-	return regexec(preg, subject, 0, NULL, 0);
+	int match;
+	char old_char = subject[length];
+	subject[length] = 0;
+	match = regexec(preg, subject, 0, NULL, 0);
+	subject[length] = old_char;
+	return match;
 #endif
 }
 
diff --git a/src/acl.c b/src/acl.c
index 89bfdf5a0..e6cbd306a 100644
--- a/src/acl.c
+++ b/src/acl.c
@@ -163,19 +163,9 @@ static void *acl_lookup_str(struct sample *smp, struct acl_expr *expr)
  */
 int acl_match_reg(struct sample *smp, struct acl_pattern *pattern)
 {
-	char old_char;
-	int ret;
-
-	old_char = smp->data.str.str[smp->data.str.len];
-	smp->data.str.str[smp->data.str.len] = 0;
-
 	if (regex_exec(pattern->ptr.reg, smp->data.str.str, smp->data.str.len) == 0)
-		ret = ACL_PAT_PASS;
-	else
-		ret = ACL_PAT_FAIL;
-
-	smp->data.str.str[smp->data.str.len] = old_char;
-	return ret;
+		return ACL_PAT_PASS;
+	return ACL_PAT_FAIL;
 }
 
 /* Checks that the pattern matches the beginning of the tested string. */