Parslet test

simple_grammar.rb

require 'parslet'

class SimpleExpressionParser < Parslet::Parser
  rule(:space)      { match['\s'].repeat(1) }
  rule(:space?)     { space.maybe }

  rule(:integer)    { match('\d').repeat(1).as(:int) >> space? }
  rule(:identifier) { match('\w').repeat(1).as(:id) >> space? }

  rule(:lparan)     { str('(') >> space? }
  rule(:rparan)     { str(')') >> space? }
  rule(:addsub)     { match['+-'].as(:op) >> space? }
  rule(:muldiv)     { match['*/'].as(:op) >> space? }

  rule(:value)      { integer | identifier | lparan >> expression >> rparan }
  rule(:term)       { value.as(:left) >> muldiv >> term.as(:right) | value }
  rule(:expression) { term.as(:left) >> addsub >> expression.as(:right) | term }

  rule(:statement)  { expression }

  rule(:input)      { space? >> statement.repeat(0) }

  root(:input)
end

puts SimpleExpressionParser.new.parse(' a * (b + 2) - 5')

# {:left=>{:left=>{:id=>"a"}, :op=>"*", :right=>{:left=>{:id=>"b"}, :op=>"+", :right=>{:int=>"2"}}}, :op=>"-", :right=>{:int=>"5"}}