deprecate using invalid names for buttons

UPGRADE-4.3.md

@@ -23,6 +23,10 @@ Config
 Form
 ----
 
+ * Using names for buttons that do not start with a letter, a digit, or an underscore is deprecated and will lead to an
+   exception in 5.0.
+ * Using names for buttons that do not contain only letters, digits, underscores, hyphens, and colons is deprecated and
+   will lead to an exception in 5.0.
  * Using the `date_format`, `date_widget`, and `time_widget` options of the `DateTimeType` when the `widget` option is
    set to `single_text` is deprecated.
 

UPGRADE-5.0.md

@@ -77,7 +77,10 @@ Finder
 
 Form
 ----
-
+ 
+ * Using names for buttons that do not start with a letter, a digit, or an underscore leads to an exception.
+ * Using names for buttons that do not contain only letters, digits, underscores, hyphens, and colons leads to an
+   exception.
  * Using the `date_format`, `date_widget`, and `time_widget` options of the `DateTimeType` when the `widget` option is
    set to `single_text` is not supported anymore.
  * The `getExtendedType()` method was removed from the `FormTypeExtensionInterface`. It is replaced by the the static

src/Symfony/Component/Form/ButtonBuilder.php

@@ -63,6 +63,18 @@ class ButtonBuilder implements \IteratorAggregate, FormBuilderInterface
 
         $this->name = $name;
         $this->options = $options;
+
+        if (\preg_match('/^([^a-z0-9_].*)?(.*[^a-zA-Z0-9_\-:].*)?$/D', $name, $matches)) {
+            if (isset($matches[1])) {
+                @trigger_error(sprintf('Using names for buttons that do not start with a letter, a digit, or an underscore is deprecated since Symfony 4.3 and will throw an exception in 5.0 ("%s" given).', $name), E_USER_DEPRECATED);
+            }
+            if (isset($matches[2])) {
+                @trigger_error(sprintf('Using names for buttons that do not contain only letters, digits, underscores ("_"), hyphens ("-") and colons (":") ("%s" given) is deprecated since Symfony 4.3 and will throw an exception in 5.0.', $name), E_USER_DEPRECATED);
+            }
+        }
+
+        // to be added in 5.0
+        // FormConfigBuilder::validateName($name);
     }
 
     /**

src/Symfony/Component/Form/CHANGELOG.md

@@ -4,6 +4,10 @@ CHANGELOG
 4.3.0
 -----
 
+ * Using names for buttons that do not start with a letter, a digit, or an underscore is deprecated and will lead to an
+   exception in 5.0.
+ * Using names for buttons that do not contain only letters, digits, underscores, hyphens, and colons is deprecated and
+   will lead to an exception in 5.0.
  * deprecated using the `date_format`, `date_widget`, and `time_widget` options of the `DateTimeType` when the `widget`
    option is set to `single_text`
  * added `block_prefix` option to `BaseType`.

src/Symfony/Component/Form/Tests/ButtonBuilderTest.php

@@ -28,7 +28,6 @@ class ButtonBuilderTest extends TestCase
             ['foo'],
             ['0'],
             [0],
-            ['button[]'],
         ];
     }
 
@@ -40,6 +39,14 @@ class ButtonBuilderTest extends TestCase
         $this->assertInstanceOf('\Symfony\Component\Form\ButtonBuilder', new ButtonBuilder($name));
     }
 
+    /**
+     * @group legacy
+     */
+    public function testNameContainingIllegalCharacters()
+    {
+        $this->assertInstanceOf('\Symfony\Component\Form\ButtonBuilder', new ButtonBuilder('button[]'));
+    }
+
     public function getInvalidNames()
     {
         return [