allow button names to start with uppercase letter

UPGRADE-4.3.md

@@ -96,7 +96,7 @@ Form
 ----
 
  * Using the `format` option of `DateType` and `DateTimeType` when the `html5` option is enabled is deprecated.
- * Using names for buttons that do not start with a lowercase letter, a digit, or an underscore is deprecated and will lead to an
+ * 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.

UPGRADE-5.0.md

@@ -110,7 +110,7 @@ Form
 ----
 
  * Removed support for using the `format` option of `DateType` and `DateTimeType` when the `html5` option is enabled.
- * Using names for buttons that do not start with a lowercase letter, a digit, or an underscore leads to an exception.
+ * 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

src/Symfony/Component/Form/ButtonBuilder.php

@@ -61,7 +61,7 @@ class ButtonBuilder implements \IteratorAggregate, FormBuilderInterface
         $this->name = $name;
         $this->options = $options;
 
-        if (preg_match('/^([^a-z0-9_].*)?(.*[^a-zA-Z0-9_\-:].*)?$/D', $name, $matches)) {
+        if (preg_match('/^([^a-zA-Z0-9_].*)?(.*[^a-zA-Z0-9_\-:].*)?$/D', $name, $matches)) {
             if (isset($matches[1])) {
                 @trigger_error(sprintf('Using names for buttons that do not start with a lowercase 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);
             }

src/Symfony/Component/Form/CHANGELOG.md

@@ -6,7 +6,7 @@ CHANGELOG
 
  * added a `symbol` option to the `PercentType` that allows to disable or customize the output of the percent character
  * Using the `format` option of `DateType` and `DateTimeType` when the `html5` option is enabled is deprecated.
- * Using names for buttons that do not start with a lowercase letter, a digit, or an underscore is deprecated and will lead to an
+ * 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.

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

@@ -47,14 +47,6 @@ class ButtonBuilderTest extends TestCase
         $this->assertInstanceOf('\Symfony\Component\Form\ButtonBuilder', new ButtonBuilder('button[]'));
     }
 
-    /**
-     * @group legacy
-     */
-    public function testNameStartingWithIllegalCharacters()
-    {
-        $this->assertInstanceOf('\Symfony\Component\Form\ButtonBuilder', new ButtonBuilder('Button'));
-    }
-
     public function getInvalidNames()
     {
         return [