++stevieb for helpful suggestions. Also, Anonymous Monk makes a good point that a list of test cases would be very helpful. That is, the code that stevieb posted works for the cases that you have mentioned in your post and replies...but it may fail on other cases that you have failed to mention here.

This is a case where the YAPE::Regex::Explain module can be helpful (see item 9 in Basic debugging checklist).

Here is the YAPE::Regex::Explain output for your original regex: 
The regular expression:

(?-imsx:^https?://(?!www\.)robidu\.de/)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  ^                        the beginning of the string
----------------------------------------------------------------------
  http                     'http'
----------------------------------------------------------------------
  s?                       's' (optional (matching the most amount
                           possible))
----------------------------------------------------------------------
  ://                      '://'
----------------------------------------------------------------------
  (?!                      look ahead to see if there is not:
----------------------------------------------------------------------
    www                      'www'
----------------------------------------------------------------------
    \.                       '.'
----------------------------------------------------------------------
  )                        end of look-ahead
----------------------------------------------------------------------
  robidu                   'robidu'
----------------------------------------------------------------------
  \.                       '.'
----------------------------------------------------------------------
  de/                      'de/'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

[download]
Here is the YAPE::Regex::Explain output for stevieb's regex: 
The regular expression:

(?-imsx:^https?://.*?\.?(?<!www\.)robidu\.de/)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  ^                        the beginning of the string
----------------------------------------------------------------------
  http                     'http'
----------------------------------------------------------------------
  s?                       's' (optional (matching the most amount
                           possible))
----------------------------------------------------------------------
  ://                      '://'
----------------------------------------------------------------------
  .*?                      any character except \n (0 or more times
                           (matching the least amount possible))
----------------------------------------------------------------------
  \.?                      '.' (optional (matching the most amount
                           possible))
----------------------------------------------------------------------
  (?<!                     look behind to see if there is not:
----------------------------------------------------------------------
    www                      'www'
----------------------------------------------------------------------
    \.                       '.'
----------------------------------------------------------------------
  )                        end of look-behind
----------------------------------------------------------------------
  robidu                   'robidu'
----------------------------------------------------------------------
  \.                       '.'
----------------------------------------------------------------------
  de/                      'de/'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

[download]

In reply to Re: regexp: Mind-boggling negative assertions... by kevbot
in thread regexp: Mind-boggling negative assertions... by Robidu

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