Although CCK automatically does some basic validation on your fields that you add to your Drupal content types, there are some cases where you'd like to do some additional validation for your site. One use case that I ran into recently was a basic text field that was being used to house hyperlinks for one of my websites. The text field had already been in place and working perfectly for months. Rather than do something drastic like replacing the field altogether with a field provided by the "Link" module, I decided to do a hook_form_alter to add in my own custom validation function.

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