regex lookahead problems with greedy quantifier -

The following formats need to be supported

After 3 digits, three non- After repeating characters

  123 123 A 123 a v 123 CER  
 
 
 
 / pre> < P> Invalid format: 
 
  123 A 123 AA 123A - when done with a location  
 
 - I may not have necessarily That is, by making a disturbance with Look-Eyed: 
 
  ^ ([0- 9] {3}) # - the first 3 digits (\ s (? = [ACERV]) ([ACERV]) # - Cave Allow space followed by ACERV (?!!)? (? = [ACERV] {0,1}) ([ACERV] {0,1}) # - Do not allow repeat letters first?! # 3 - Do not allow the first grade to be repeated (?! \ 4) # - Do not allow second to repeat again (?! \ S) # - Do not allow back space (? = [ACERV] {0}} ([ACERV] {0,1}). [0- 9] {3} $  
 
 When the visible (?! \ 4) is added, then this valid format fails to match on 123 A - on quantifiers ( ?! \ 4) to modify (?! \ 4) * or (?! \ 4)? 123 A allows the match but allows it to be repeated 1 or 2 times. 
 

Not entirely sure of requirements, it works on your samples

  # ^ (? I) \ d {3} (?: [] (?: ([ACERV]) [?]? (?! [ACERV] * \ 1) {1 # 3 digits (?: # Cluster GRP, Character Block (optional) [] #space, 3} (? & Lt;! [])) $ ^ # Bol (? I) # case insensitive modifier \ essential (?: # Cluster GRP ) # (1), Capture single characters [Aceerve] []? # [], Optional (?! # Negative lookhead [Aceerve] * # Many [ACHEW] or [] have been captured in group 1 # 1 needs, this claim has failed) # and negative lookhead) {1,3} # and cluster groups, 1-3 [Aquava] characters (? & Lt ;!) # No dropdown at the end []? # Adjusted to replace cluster GRP, Character Block (optional) $ # EOL  

Update -

  # ^ (? I) \ d {3} (?!! * [] $) (?: [] (?: ([ACERV]) []? (?!? [ACERV] * \ 1)) {1,3}) $ ^ # BOL (? I) # Case insensitive modifier \ d {3} # 3 digits (?! *. [$ $] # No dropdown at the end []: # Cluster GRP, Character Block (optional) [] # space, Required (?: # Cluster Group ([ACERWEB]) # (1), Capture single characters [Aceerve] []? # []! # Negative lookhead [Aquav] # # To get to know # 1 [Ekve] or [] Group 1 is needed to know that # is found # , Surrender fails) # End negative attitudes) {1,3} # End Cluster GRP, 1-3 (ACWRV) meets characters)? # End Cluster GRP, Character Block (optional) $ # EOL