Source

File: wp-includes/class-wp-embed.php

public function autoembed_callback( $match ) {
		$oldval              = $this->linkifunknown;
		$this->linkifunknown = false;
		$return              = $this->shortcode( array(), $match[2] );
		$this->linkifunknown = $oldval;

		return $match[1] . $return . $match[3];
	}