/CCGG          # Match starting at DNA sequence CCGG
  (          
    (?:        
      (?!CCGG) #   make sure we're not finding duplicates mid-stream
      .             #   accept any character
    )*?            # any number of times BUT not greedily <====
  )          
  AATT         # and ending at AATT
/x;

##</code><code>##

/CCGG   
  (          
    (?:        
      (?!CCGG)  
      .        
    ){50,100}?        # <====
   )         
  AATT  # and ending at AATT
/x;

##</code><code>##

/CCGG   
  (          
    (?:        
      (?!AATT|CCGG) #     <=============
      .       # 
    ){50,100}?        # Here the "?" is not required but I'm anal
  )         # 
  AATT  # 
/x;