diff --git a/src/Form/GridHeightSelectorForm.php b/src/Form/GridHeightSelectorForm.php
index 49ed171..52d3173 100644
--- a/src/Form/GridHeightSelectorForm.php
+++ b/src/Form/GridHeightSelectorForm.php
@@ -52,6 +52,63 @@ class GridHeightSelectorForm extends ConfigFormBase {
   /**
    * {@inheritdoc}
    */
+  public function validateForm(array &$form, FormStateInterface $form_state) {
+    // Check class selectors.
+    $class_or_not = $form_state->getValue(array('grid_height_equalizer_selectors'));
+    $invalid_class = $duplicate_class = $invalid_character = FALSE;
+    if (!empty($class_or_not)) {
+      $classes = preg_split('/\R/', $class_or_not);
+      foreach ($classes as $key => $class_name) {
+        // Check class selector starts with a dot.
+        if (strpos($class_name, '.') !== 0 || (substr($class_name, -1) == '.')) {
+          $invalid_class = TRUE;
+        }
+        // Check selectors has space.
+        if (preg_match('/[ ]/', $class_name)) {
+          $space_seperator = TRUE;
+        }
+        if (preg_match('/[\'\/~`\!@#\$%\^&\*\(\)_\+=\{\}\[\]\|;:"\<\>,\?\\\]/', $class_name)) {
+          $invalid_character = TRUE;
+        }
+      }
+
+      // Check and listout the duplicate class names added in the form.
+      $error_classes = '';
+      $classes_count = array_count_values($classes);
+      foreach ($classes_count as $key => $value) {
+        if ($value > 1) {
+          $error_classes .= $key . ', ';
+        }
+      }
+
+      $error_classes = rtrim($error_classes, ', ');
+      if (!empty($error_classes)) {
+        $duplicate_class = TRUE;
+      }
+
+      if ($duplicate_class == TRUE || $invalid_class == TRUE ||
+        $space_seperator == TRUE || $invalid_character == TRUE) {
+        $message = '';
+        if ($duplicate_class == TRUE) {
+          $message .= 'Duplicate selectors found :- ' . $error_classes . '.';
+        }
+        if ($invalid_class == TRUE) {
+          $message .= ' Selectors should be a class name with a dot. For example ".article-list" .';
+        }
+        if ($space_seperator == TRUE) {
+          $message .= ' Supports only one selector in a line without space.';
+        }
+        if ($invalid_character == TRUE) {
+          $message .= ' Special characters are not allowed in the selectors except "." and "-".';
+        }
+        $form_state->setErrorByName('grid_height_equalizer_selectors', t('@message', ['@message' => $message]));
+      }
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
   public function submitForm(array &$form, FormStateInterface $form_state) {
     $this->config('gridheightnormalizer.settings')
       ->set('grid_height_equalizer_selectors', $form_state->getValue(array('grid_height_equalizer_selectors')))