diff --git a/lib/filterx/expr-regexp-subst.c b/lib/filterx/expr-regexp-subst.c
index 4dc1ad2ba..ba6d2bb95 100644
--- a/lib/filterx/expr-regexp-subst.c
+++ b/lib/filterx/expr-regexp-subst.c
@@ -295,6 +295,13 @@ _extract_optional_flags(FilterXFuncRegexpSubst *self, FilterXFunctionArgs *args,
   return TRUE;
 }
 
+static gboolean
+_contains_match_grp_ref(gchar *str)
+{
+  gchar *close = NULL;
+  return _next_matchgrp_ref(str, &close) != NULL;
+}
+
 static gboolean
 _extract_subst_args(FilterXFuncRegexpSubst *self, FilterXFunctionArgs *args, GError **error)
 {
@@ -366,7 +373,8 @@ filterx_function_regexp_subst_new(FilterXFunctionArgs *args, GError **error)
   self->super.super.deinit = _subst_deinit;
   self->super.super.free_fn = _subst_free;
 
-  reset_flags(&self->flags, FLAG_VAL(FILTERX_FUNC_REGEXP_SUBST_FLAG_JIT));
+  reset_flags(&self->flags, FLAG_VAL(FILTERX_FUNC_REGEXP_SUBST_FLAG_JIT) | FLAG_VAL(
+                FILTERX_FUNC_REGEXP_SUBST_FLAG_GROUPS));
   if (!_extract_subst_args(self, args, error) ||
       !filterx_function_args_check(args, error))
     goto error;