diff --git a/framework/CHANGELOG.md b/framework/CHANGELOG.md
index 0882930915..6592608453 100644
--- a/framework/CHANGELOG.md
+++ b/framework/CHANGELOG.md
@@ -17,6 +17,7 @@ Yii Framework 2 Change Log
 - Enh #14081: Added `yii\caching\CacheInterface` to make custom cache extensions adoption easier (silverfire)
 - Chg #14286: Used primary inputmask package name instead of an alias (samdark)
 - Enh #14298: The default response formatter configs defined by `yii\web\Response::defaultFormatters()` now use the array syntax (brandonkelly)
+- Bug #14304: Fixed `yii\validators\UniqueValidator` and `yii\validators\ExistValidator` to skip prefixes in case expressions are used (samdark)
 - Bug #14341: Fixed regression in error handling introduced by fixing #14264 (samdark)
 
 2.0.12 June 05, 2017
diff --git a/framework/validators/ExistValidator.php b/framework/validators/ExistValidator.php
index 91b78b33dc..a582701a46 100644
--- a/framework/validators/ExistValidator.php
+++ b/framework/validators/ExistValidator.php
@@ -221,10 +221,16 @@ class ExistValidator extends Validator
         }
         $prefixedConditions = [];
         foreach ($conditions as $columnName => $columnValue) {
-            $prefixedColumn = "{$alias}.[[" . preg_replace(
+            if (strpos($columnName, '(') === false) {
+                $prefixedColumn = "{$alias}.[[" . preg_replace(
                     '/^' . preg_quote($alias) . '\.(.*)$/',
                     '$1',
                     $columnName) . ']]';
+            } else {
+                // there is an expression, can't prefix it reliably
+                $prefixedColumn = $columnName;
+            }
+
             $prefixedConditions[$prefixedColumn] = $columnValue;
         }
         return $prefixedConditions;
diff --git a/framework/validators/UniqueValidator.php b/framework/validators/UniqueValidator.php
index d4d1b61ba7..5b569a279c 100644
--- a/framework/validators/UniqueValidator.php
+++ b/framework/validators/UniqueValidator.php
@@ -288,10 +288,16 @@ class UniqueValidator extends Validator
         }
         $prefixedConditions = [];
         foreach ($conditions as $columnName => $columnValue) {
-            $prefixedColumn = "{$alias}.[[" . preg_replace(
+            if (strpos($columnName, '(') === false) {
+                $prefixedColumn = "{$alias}.[[" . preg_replace(
                     '/^' . preg_quote($alias) . '\.(.*)$/',
                     '$1',
                     $columnName) . ']]';
+            } else {
+                // there is an expression, can't prefix it reliably
+                $prefixedColumn = $columnName;
+            }
+
             $prefixedConditions[$prefixedColumn] = $columnValue;
         }
         return $prefixedConditions;
diff --git a/tests/framework/validators/ExistValidatorTest.php b/tests/framework/validators/ExistValidatorTest.php
index 385c723d3f..b91ab41808 100644
--- a/tests/framework/validators/ExistValidatorTest.php
+++ b/tests/framework/validators/ExistValidatorTest.php
@@ -11,6 +11,7 @@ use Yii;
 use yii\base\Exception;
 use yii\validators\ExistValidator;
 use yiiunit\data\ar\ActiveRecord;
+use yiiunit\data\ar\Document;
 use yiiunit\data\ar\Order;
 use yiiunit\data\ar\OrderItem;
 use yiiunit\data\validators\models\ValidatorTestMainModel;
@@ -190,4 +191,20 @@ abstract class ExistValidatorTest extends DatabaseTestCase
 
         OrderItem::$tableName = $oldTableName;
     }
+
+    /**
+    * Test expresssion in targetAttribute
+    * @see https://github.com/yiisoft/yii2/issues/14304
+    */
+   public function testExpresionInAttributeColumnName()
+   {
+       $val = new ExistValidator([
+           'targetClass' => OrderItem::className(),
+           'targetAttribute' => ['id' => 'COALESCE(order_id, 0)'],
+       ]);
+
+       $m = new Order(['id' => 1]);
+       $val->validateAttribute($m, 'id');
+       $this->assertFalse($m->hasErrors('id'));
+   }
 }
diff --git a/tests/framework/validators/UniqueValidatorTest.php b/tests/framework/validators/UniqueValidatorTest.php
index 2ceb40b32a..84e2af5f25 100644
--- a/tests/framework/validators/UniqueValidatorTest.php
+++ b/tests/framework/validators/UniqueValidatorTest.php
@@ -11,6 +11,7 @@ use Yii;
 use yii\validators\UniqueValidator;
 use yiiunit\data\ar\ActiveRecord;
 use yiiunit\data\ar\Customer;
+use yiiunit\data\ar\Document;
 use yiiunit\data\ar\Order;
 use yiiunit\data\ar\OrderItem;
 use yiiunit\data\ar\Profile;
@@ -412,4 +413,25 @@ abstract class UniqueValidatorTest extends DatabaseTestCase
         $validator->validateAttribute($model, 'id');
         $this->assertFalse($model->hasErrors());
     }
+
+    /**
+     * Test expresssion in targetAttribute
+     * @see https://github.com/yiisoft/yii2/issues/14304
+     */
+    public function testExpresionInAttributeColumnName()
+    {
+        $validator = new UniqueValidator([
+            'targetAttribute' => [
+                'title' => 'LOWER(title)',
+            ],
+        ]);
+        $model = new Document();
+        $model->id = 42;
+        $model->title = 'Test';
+        $model->content = 'test';
+        $model->version = 1;
+        $model->save(false);
+        $validator->validateAttribute($model, 'title');
+        $this->assertFalse($model->hasErrors(), 'There were errors: ' . json_encode($model->getErrors()));
+    }
 }