Arithmetic expression parser with StringScanner in Ruby, inspired by parser combinators.

parser-strscan.rb

require 'strscan'

class Parser

  def initialize string
    @scanner = StringScanner.new string
  end

  def self.eval string
    self.new(string).expr
  end

  def nat
    @scanner.scan(/\d+/)&.to_i
  end

  def spaces
    @scanner.scan(/\s*/)
  end

  def sym str
    @scanner.scan(Regexp.compile(Regexp.escape(str)))
  end

  def token &block
    spaces && (result = block.call) && spaces
    result
  end

  def symbol str
    token { sym str }
  end

  def natural
    token { nat }
  end

  def expr
    if t = term
      if symbol("+") && (e = expr)
        t + e
      else
        t
      end
    end
  end

  def term
    if f = factor
      if symbol("*") && (t = term)
        f * t
      else
        f
      end
    end
  end

  def factor
    result =
      symbol("(") &&
        (e = expr) &&
        symbol(")")
    if result
      e
    else
      natural
    end
  end

end

if __FILE__ == $PROGRAM_NAME
  puts "Expected: 9876543210"
  p Parser.eval("9876543210")

  puts "\nExpected: 9"
  p Parser.eval("1+ 2* 4")    #=> 9

  puts "\nExpected: 12"
  p Parser.eval("(1 +2) *4 ") #=> 12

  puts "\nExpected: 9"
  p Parser.eval("(1 +(2 *4))") #=> 9

  puts "\nExpected: 6"
  p Parser.eval("1 + 2 + 3")  #=> 6

  puts "\nExpected: 24"
  p Parser.eval("2*3*4")      #=> 24

  puts "\nExpected: 42"
  p Parser.eval("2*((3+4) * 3)") #=> 42
end