regex  -> or { regex.value = or.value }

 or     -> cat { or'.inh=cat.value } or' { or.value = or'.value }

 or'    -> '|' or { or'.value = or'.inh + or.value }
          | € { or'.value= or'.inh }

 cat    -> star { cat'.inh = star.value } cat' { cat.value = cat'.inh }

 cat'   -> cat { cat'.inh.next = cat.value ; cat'.value = cat.value }
          | € { cat'.value = cat'.inh }

 star   -> factor { star.value = factor.value }
          | factor* { factor.value.next = factor.value ; star.value = factor.value }

 factor -> letter { factor.value = letter }
          | (regex) { factor.value = regex.value }

 letter -> [a-zA-Z0-9]