search block form not accepting special characters in drupal 7
The error has nothing to do with special characters (e.g. "$", "@"). It pertains to the length of your keywords. By default, Drupal core search restricts minimum keyword size to 3; any query less than 3 characters will trigger the error (e.g. "$9"). 

If you really need search to include 2/1 character queries, the search settings in /admin/config/search/settings under the Minimum word length to index option will allow you to do this.

Note: This will expand out the size of your search index (catching common words like "an" &amp; "a"). If you have a large site with thousands of nodes you shouldn't do this.