scan

# File lib/erb.rb, line 517
        def scan
          stag_reg = /(.*?)(#{stags.join('|')}|\z)/
          etag_reg = /(.*?)(#{etags.join('|')}|\z)/
          scanner = StringScanner.new(@src)
          while ! scanner.eos?
            scanner.scan(@stag ? etag_reg : stag_reg)
            yield(scanner[1])
            yield(scanner[2])
          end
        end