Test custom filters before adding them

Several people have reported bugs that were caused by custom filters without properly escaped regular expressions:

http://drupal.org/node/160208
http://drupal.org/node/188305
http://drupal.org/node/71838
http://drupal.org/node/141076

It would be helpful if, after submitting a custom regular expression but before saving it, an attempt was made to use the regular expression and test the return code. If it returns an error, the regular expression should not be saved and a warning should be given to the admin who is adding it - prompting them to fix it first.