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
----------------------------------------------------------------------

##</code><code>##

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
----------------------------------------------------------------------