Source

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

public function get_provider( $url, $args = '' ) {
		$args = wp_parse_args( $args );

		$provider = false;

		if ( ! isset( $args['discover'] ) ) {
			$args['discover'] = true;
		}

		foreach ( $this->providers as $matchmask => $data ) {
			list( $providerurl, $regex ) = $data;

			// Turn the asterisk-type provider URLs into regex.
			if ( ! $regex ) {
				$matchmask = '#' . str_replace( '___wildcard___', '(.+)', preg_quote( str_replace( '*', '___wildcard___', $matchmask ), '#' ) ) . '#i';
				$matchmask = preg_replace( '|^#http\\\://|', '#https?\://', $matchmask );
			}

			if ( preg_match( $matchmask, $url ) ) {
				$provider = str_replace( '{format}', 'json', $providerurl ); // JSON is easier to deal with than XML.
				break;
			}
		}

		if ( ! $provider && $args['discover'] ) {
			$provider = $this->discover( $url );
		}

		return $provider;
	}