I had to go through many, many iterations to get my regex working correctly, and each time I modified and saved my filter, it would get automatically disabled. After saving it would appear as disabled on admin/structure/flexifilters. Required 2 extra steps and page loads to get re-enabled.

I'm using flexifilters 7.x-1.0-beta1 on drupal 7.9

This seems related to usability when heavily editing a filter like this old issue