I'm trying to come up with a regular expression to validate a comma-separated list of tokens.  The rules are:

- The token may contain any characters (i.e. alphanumerics, punctuations, ...).
- There may not be any spaces between the comma and the tokens.
- The list must not end with a comma.

Valid lists:

A
#$4
ABC
ABC,XY"5,t5\6!@,$%45
Jo*hn,M!^-A,78A$#@2,FGer%^.6

Invalid lists:

A,B,
AG*,HIG,
ABC,   DEF,       GHI

Thank you in advance for your help.

^(?=(?!.*(?: ,|, |.*,$))).*$

Test URL:

http://www.myregextester.com/?r=228

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Thank you for the reply.  I'm a regex newbie and I really want to find out why it works.  Let me go do some reading.

The regular expression:

^(?=(?!.*(?: ,|, |.*,$))).*$

matches as follows:
 
NODE                     EXPLANATION
----------------------------------------------------------------------
  ^                        the beginning of the string
----------------------------------------------------------------------
  (?=                      look ahead to see if there is:
----------------------------------------------------------------------
    (?!                      look ahead to see if there is not:
----------------------------------------------------------------------
      .*                       any character except \n (0 or more
                               times (matching the most amount
                               possible))
----------------------------------------------------------------------
      (?:                      group, but do not capture:
----------------------------------------------------------------------
         ,                       ' ,'
----------------------------------------------------------------------
       |                        OR
----------------------------------------------------------------------
        ,                        ', '
----------------------------------------------------------------------
       |                        OR
----------------------------------------------------------------------
        .*                       any character except \n (0 or more
                                 times (matching the most amount
                                 possible))
----------------------------------------------------------------------
        ,                        ','
----------------------------------------------------------------------
        $                        before an optional \n, and the end
                                 of a "line"
----------------------------------------------------------------------
      )                        end of grouping
----------------------------------------------------------------------
    )                        end of look-ahead
----------------------------------------------------------------------
  )                        end of look-ahead
----------------------------------------------------------------------
  .*                       any character except \n (0 or more times
                           (matching the most amount possible))
----------------------------------------------------------------------
  $                        before an optional \n, and the end of
                            the string
----------------------------------------------------------------------

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Thank you for the explanation.