A json parser

jsonsttn.rb

require "strscan"
class JSONParser
	AST = Struct.new(:value)
	def parse(input)
		@input = StringScanner.new(input)
		parse_value.value
	ensure
		@input.eos? or error("Unexpected data")
	end
	
	private
	
	#comment is back
	def parse_value
		trim_space
		parse_object or
		parse_array or
		parse_string or
		parse_number or
		parse_keyword or
		error("Invalid Data")
	ensure
		trim_space	
	end
	
	#Parses colon separated object hashes
	def parse_object
		if @input.scan(/\s*\{/)
			obj = Hash.new
			more_parts = false
			while key = parse_string
				@input.scan(/\s*:\s*/) or error("Expected : separator")
				obj[key.value] = parse_value.value
				more_parts = @input.scan(/\s*,\s*/) or break
			end
			error("Missing object pair") if more_parts
			@input.scan(/\s*}/) or error("Unclosed object")
			AST.new(obj)
		else
			false
		end
	end
	
	def parse_array
		if @input.scan(/\s*\[/) #Arrays start with [
			a = Array.new
			more_items = false
			while current = parse_value
			a << current.value
			more_items = @input.scan(/\s*,\s*/) or break
			end
			error("Missing value") if more_items
			@input.scan(/s*\]/) or error("Unclosed array")
			AST.new(a)
		end
	end
			
	
	#Parses string objects
	def parse_string
		if @input.scan(/"/)
			s = String.new
			while current = parse_string_content || parse_string_escape
				s << current.value
			end
			@input.scan(/"/) or error('Unclosed String')
			AST.new(s)
		else
			false
		end
	end
	
	def parse_string_content
		@input.scan(/[^\\"]+/) and AST.new(@input.matched)
	end
	
	def parse_string_escape
		if @input.scan(%r{\\["\\/]}) #slashes and quotations
			AST.new(@input.matched[-1])
		elsif @input.scan(/\\[bfnrt]/) #newlines, tabs, etc
			AST.new(eval(%Q{"#{@input.matched}"}))
		elsif @input.scan(/\\u[0-9a-fA-F]{4}/) #Hex integers
			AST.new([Integer("0x#{@input.matched[2..-1]}")].pack("U"))
		else
			false
		end
	end
	
	def parse_number
		@input.scan(/-?(?:0|[1-9]\d*)(?:\.\d+)?(?:[eE][+-]?\d+)?\b/) and
		AST.new(eval(@input.matched))
	end
	
	def parse_keyword
		@input.scan(/\b(?:true|false|null)\b/) and
		AST.new(eval(@input.matched.sub("null","nil")))
	end
	
	def trim_space
		@input.scan(/\s+/)
	end
	
	def error(message)
			raise "#{message}:  #{@input.peek(@input.string.length)}"
	end
end