code: def evaluate(formula, fail_result = 0)
          if value = eval(formula)
            last_match = $1
            (formula =~ /^c\(?\d+=~/) ? last_match : value
          else
            fail_result
          end   
        end
docs: Evaluate a formula
   Example invokation
       evaluate("n1+n2", 0)
       evaluate("c1+c2", "failed")
       evaluate("c0=~/\\.(\\d{4})/", "0")