Source

File: wp-includes/class-wp-customize-manager.php

public function _validate_external_header_video( $validity, $value ) {
		$video = esc_url_raw( $value );
		if ( $video ) {
			if ( ! preg_match( '#^https?://(?:www\.)?(?:youtube\.com/watch|youtu\.be/)#', $video ) ) {
				$validity->add( 'invalid_url', __( 'Please enter a valid YouTube URL.' ) );
			}
		}
		return $validity;
	}