diff --git a/UPGRADE-4.3.md b/UPGRADE-4.3.md
index 20b970c15a..27027b8ba4 100644
--- a/UPGRADE-4.3.md
+++ b/UPGRADE-4.3.md
@@ -61,7 +61,7 @@ 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 letter, a digit, or an underscore is deprecated and will lead to an
+ * Using names for buttons that do not start with a lowercase 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.
diff --git a/UPGRADE-5.0.md b/UPGRADE-5.0.md
index e16fb86c79..d107936658 100644
--- a/UPGRADE-5.0.md
+++ b/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 letter, a digit, or an underscore leads to an exception.
+ * 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 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
diff --git a/src/Symfony/Component/Form/ButtonBuilder.php b/src/Symfony/Component/Form/ButtonBuilder.php
index 19196729f1..f78ae4ded1 100644
--- a/src/Symfony/Component/Form/ButtonBuilder.php
+++ b/src/Symfony/Component/Form/ButtonBuilder.php
@@ -63,7 +63,7 @@ class ButtonBuilder implements \IteratorAggregate, FormBuilderInterface
 
         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);
+                @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).', $name), E_USER_DEPRECATED);
             }
             if (isset($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);
diff --git a/src/Symfony/Component/Form/CHANGELOG.md b/src/Symfony/Component/Form/CHANGELOG.md
index e41c46e907..cf7b8cd8cc 100644
--- a/src/Symfony/Component/Form/CHANGELOG.md
+++ b/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 letter, a digit, or an underscore is deprecated and will lead to an
+ * Using names for buttons that do not start with a lowercase 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.
diff --git a/src/Symfony/Component/Form/Tests/ButtonBuilderTest.php b/src/Symfony/Component/Form/Tests/ButtonBuilderTest.php
index e987ef7eab..b10cd063d6 100644
--- a/src/Symfony/Component/Form/Tests/ButtonBuilderTest.php
+++ b/src/Symfony/Component/Form/Tests/ButtonBuilderTest.php
@@ -47,6 +47,14 @@ 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 [