I am writing a parser in python that will parse the given grammar into an Abstract Syntax Tree. I already have a scanner built using regular expressions. I'm having a lot of trouble actually starting the parser portion. My main issue is how do I tap into my token list after the file has been read, additionally I'm not really sure how to code the grammar so that it will be output in AST. Can anyone help get me started and on the right track? Thanks(Python code and Grammar below).

import re import sys f = open("scannertestdata.txt","r") string = f.read() #string = sys.stdin.read() use later when submitting scanner=re.Scanner([ (r"(var|fun|if|else|return|read|write|not|or|and)", lambda scanner,token:("KEYWORD: ", token)), (r"\d+\.\d*",lambda scanner,token:("REAL",token)), (r"\d+",lambda scanner,token:("INTEGER", token)), (r"[a-zA-Z0-9]+",lambda scanner,token:("IDENTIFIER", token)), (r"(\(|\)|{|}|,|\+|\-|\*|/|%|:=|!=|=|)",lambda scanner,token:("PUNCTUATION", token)), (r"(#t|#f)+",lambda scanner,token:("BOOLEAN", token)), (r"\s+",None),# Skips white space (r'.', lambda scanner,token:None)#Skips invalid token ]) results, remainder=scanner.scan(string) print(results[0] ) 

program id bool literal int literal real literal constant cmds BOOLEAN> KINTEGER> CREAL> bool_literal int literal | real_literal any IDENTIFIER token assignment cmd cmds id := expr ; Expressions are based on Pascal grammar: https:/www.cs.utexas.edu/usersovak/grammar.html simple-expr [rel-op simple-expr] expr simple-expr term factor [sign op 1 term faddop term constant l id I not-op factor | ( expr) expr { , expr) not factor mul op factor exprs not op sign_op add-op mul-op rel op not, or, and were scanned as keywords, but they are used as operators program id bool literal int literal real literal constant cmds BOOLEAN> KINTEGER> CREAL> bool_literal int literal | real_literal any IDENTIFIER token assignment cmd cmds id := expr ; Expressions are based on Pascal grammar: https:/www.cs.utexas.edu/usersovak/grammar.html simple-expr [rel-op simple-expr] expr simple-expr term factor [sign op 1 term faddop term constant l id I not-op factor | ( expr) expr { , expr) not factor mul op factor exprs not op sign_op add-op mul-op rel op not, or, and were scanned as keywords, but they are used as operators