Migrating old posts to new wp, how to extrapolate infos from the post body to custom fields?

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The short answer is that you Loop over all of you posts and use (probably) preg_match to extract information.

The long answer is that this could be pretty tricky, depending on how consistent your data entry is. Here is an example:

global $wpdb;
$sql = "SELECT ID,post_content FROM {$wpdb->posts}";
$allp = $wpdb->get_results($sql);
foreach ($allp as $p) {
  // Deal with price
  $pat="/price:[0-9]+\$/";
  $price = preg_match($pat,$p->post_content,$matches);
  if (!empty($matches[0])) {
    list($key,$value) = explode(':',$matches[0]);
    update_post_meta($p->ID, $key, $value, $value);
    $new_content = preg_replace($pat,'',$p->post_content);
    //     var_dump($new_content);
  }
  // end of price

  // update content if needed
  if (!empty($new_content)) {
    $post = array(
      'ID' => $p->ID,
      'post_content' => $new_content
    );
    wp_update_post($post);
  }

}

That will convert the “price” data you listed above– price:200$– and put it into a meta field (*_postmeta) under the key “price” with a value of “200$”. I’d recommend not including the $ in the value so you can sort numerically if you ever want to. There are a number o questions here asking how to sort alphanumeric data numerically, and you can’t. You need to solve that as the data goes into the database.

You will need a similar regex for each of the bits of information you want to pull out of post_content.

Be very careful. It is easy to get the regex wrong, and there is no ‘undo’. Work on a development server with a copied, and disposable, database until you know you have things right.

Reference:

http://php.net/manual/en/function.preg-match.php
http://www.php.net/manual/en/function.preg-replace.php
http://codex.wordpress.org/Function_Reference/update_post_meta
http://codex.wordpress.org/Function_Reference/wp_update_post