Add test for block template name validation with underscores

src/wp-includes/class-wp-block-templates-registry.php

@@ -50,7 +50,7 @@ public function register( $template_name, $args = array() ) {
 		} elseif ( preg_match( '/[A-Z]+/', $template_name ) ) {
 			$error_message = __( 'Template names must not contain uppercase characters.' );
 			$error_code    = 'template_name_no_uppercase';
-		} elseif ( ! preg_match( '/^[a-z0-9-]+\/\/[a-z0-9-]+$/', $template_name ) ) {
+		} elseif ( ! preg_match( '/^[a-z0-9_\-]+\/\/[a-z0-9_\-]+$/', $template_name ) ) {
 			$error_message = __( 'Template names must contain a namespace prefix. Example: my-plugin//my-custom-template' );
 			$error_code    = 'template_no_prefix';
 		} elseif ( $this->is_registered( $template_name ) ) {

tests/phpunit/tests/block-templates/WpBlockTemplatesRegistry.php

@@ -267,4 +267,51 @@ public function test_unregister() {
 		$this->assertEquals( $template, $unregistered_template, 'Unregistered template should be the same as the registered one.' );
 		$this->assertFalse( self::$registry->is_registered( $template_name ), 'Template should not be registered after unregistering.' );
 	}
+
+	/**
+	 * Data provider for test_template_name_validation.
+	 *
+	 * @return array[] Test data.
+	 */
+	public static function data_template_name_validation() {
+		return array(
+			'valid_simple_name'      => array(
+				'my-plugin//my-template',
+				true,
+				'Valid template name with simple characters should be accepted',
+			),
+			'valid_with_underscores' => array(
+				'my-plugin//my_template',
+				true,
+				'Template name with underscores should be accepted',
+			),
+			'valid_cpt_archive'      => array(
+				'my-plugin//archive-my_post_type',
+				true,
+				'Template name for CPT archive with underscore should be accepted',
+			),
+		);
+	}
+
+	/**
+	 * Tests template name validation with various inputs.
+	 *
+	 * @ticket 62523
+	 *
+	 * @dataProvider data_template_name_validation
+	 *
+	 * @param string $template_name The template name to test.
+	 * @param bool   $expected      Expected validation result.
+	 * @param string $message       Test assertion message.
+	 */
+	public function test_template_name_validation( $template_name, $expected, $message ) {
+		$result = self::$registry->register( $template_name, array() );
+
+		if ( $expected ) {
+			self::$registry->unregister( $template_name );
+			$this->assertNotWPError( $result, $message );
+		} else {
+			$this->assertWPError( $result, $message );
+		}
+	}
 }