Source

File: wp-includes/formatting.php

function get_url_in_content( $content ) {
	if ( empty( $content ) ) {
		return false;
	}

	if ( preg_match( '/<a\s[^>]*?href=([\'"])(.+?)\1/is', $content, $matches ) ) {
		return esc_url_raw( $matches[2] );
	}

	return false;
}