Add target attribute to all links in post

functions.php

/*** Add target attribute to all links in post ***/
function gt_add_target_attr( $match ){
  if( strpos( $match[0], 'target=' ) === false ){

    return preg_replace( '#>$#', ' target="_blank">', $match[0] );

  } else {
    return $match[0];
  }
}
function gt_add_target_to_links( $content ) {

  $content = preg_replace_callback( '#<a.+href="(.+)".*>#mUui', 'gt_add_target_attr', $content );
  return $content;

}
add_filter( 'the_content', 'gt_add_target_to_links' );
/*** End Add target attribute to all links in post ***/