fix: migrate action expression validation to separate rule class

app/Rules/IsValidActionExpression.php

@@ -0,0 +1,48 @@
+<?php
+
+/*
+ *
+ * IsValidActionExpression.php
+ * Copyright (c) 2024 Michael Thomas
+ *
+ * This file is part of Firefly III (https://github.com/firefly-iii).
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+namespace FireflyIII\Rules;
+
+use Closure;
+use Illuminate\Contracts\Validation\ValidationRule;
+use FireflyIII\TransactionRules\Expressions\ActionExpression;
+
+class IsValidActionExpression implements ValidationRule
+{
+    /**
+     * Check that the given action expression is syntactically valid.
+     *
+     * @param  \Closure(string): \Illuminate\Translation\PotentiallyTranslatedString  $fail
+     */
+    public function validate(string $attribute, mixed $value, Closure $fail): void
+    {
+        $value ??= '';
+        $expr = new ActionExpression($value);
+
+        if (!$expr->isValid()) {
+            $fail('validation.rule_action_expression')->translate([
+                'error' => $expr->getValidationError()->getMessage()
+            ]);
+        }
+    }
+}