regex - regular expression in javascript, detecting optional arguments -

i'm doing replacements on tokenized strings below:

var mystring = "i replace following token <<start<arg1,arg2 >>> in string" or   var mystring = "the expression has optional <<start<arg1,arg2,arg3 >>> third argument"  

within token, there 2 or 3 alphanumeric strings capture , pass regex callback function:

mystring.replace(regexexpression, function(m, arg1, arg2, arg3) {     return foo(arg1, arg2, arg3);  }); 

what regex expression match both versions of mystring, capture arg1, arg2, , optional arg3?

description

try put each arg group 1 2 or 3 respectively, see regexplanet sample

• each attribute group starts <<start< , ends >>>
• will match between 2 , 3 groups of comma delimited substrings between open , close tags, additional groups matched inserting (?:,([^,>]*))? between second , third groups.
• token 3 optional

<<start<([^,>]*)(?:,([^,>]*))(?:,([^>]*))?(?=>>>)