Strip HTML from Form textarea field such as Email message

@squirrel, I can think of two avenues:

• Field validation using Regex:
  Add a validation to the field which will fail when its content contains an anchor. A simple example:
  YAMLCopy

  validate:
  type: textarea
  pattern: '^(.(?!<a))*$'  # Any charactor must not be followed by '<a'
  required: true
  

  The form validation will fail on input like "This field contains an <a href="..."> in its text".

• Cleanup textarea:
  A custom plugin could respond to event onFormPrepareValidation and sanitise the data of the form. Eg. when the email form uses field 'message' as body, its field data could be cleansed as follows:
  PHPCopy

  public function onFormPrepareValidation($event) {
   $message = $event['form']->getData('message');
   $event['form']->setData('message', strip_tags($message));
  }