Example Drupal 7 input filter for inserting a node into an arbitrary formatted text field.

examplefilter.module.php

<?php

/**
 * Implements hook_filter_info().
 *
 * @link https://api.drupal.org/api/drupal/modules%21filter%21filter.api.php/function/hook_filter_info/7.x
 */
function examplefilter_filter_info() {
  $filters['examplefilter_node_embed'] = array(
    'title' => 'EPI node embed',
    'description' => 'Embed a node using its node id.',
    'process callback' => '_examplefilter_node_filter_embed',
    'tips callback' => '_examplefilter_node_filter_embed_tips',
  );

  return $filters;
}


/**
 * Tips! Provide useful information to the content editing user about how to 
 * best use this filter.
 *
 * @param $filter
 * @param $format
 * @param bool|FALSE $long
 *
 * @return string
 */
function _examplefilter_node_filter_embed_tips($filter, $format, $long = FALSE){
  return 'You can embed a node using the following snippet pattern. <code>[node nid=""]</code>';
}

/**
 * Find any usage of this filter in a given block of text and replace it with
 * the results desired. This example uses a callback to provide the node output.
 *
 * @param $text
 * @param $filter
 * @param $format
 * @param $langcode
 * @param $cache
 * @param $cache_id
 *
 * @return mixed
 */
function _examplefilter_node_filter_embed($text, $filter, $format, $langcode, $cache, $cache_id){
  $text = preg_replace_callback('|\[node nid="(.*)"\]|', '_examplefilter_node_filter_embed_replace', $text );
  return $text;
}

/**
 * Callback for preg_replace_callback that outputs a node if user has access
 * to view it.
 *
 * @param $matches
 *
 * @return string
 */
function _examplefilter_node_filter_embed_replace( $matches ){
  $nid = $matches[1];
  $node = node_load($nid);
  $output = '';

  if (node_access('view', $node)){
    $node_view = node_view($node);
    $output = drupal_render($node_view);
  }

  return $output;
}