m/
   ^[$valid_chars]   #the string must start with one of the valid chars
   (?:               #start group, but don't capture!
      ,              #could have a comma
      [$valid_chars] #and another valid char
   )*                #and that comma+valid char could repeat 0 or more times
   $                 #until the end of the string
/x                   #this is here to allow me to use all these comments