Introduction to Lex and YACC
Lex and YACC are essential tools in computer science theory for building lexical analyzers and parsers. Lex utilizes regular expressions to build a lexer, while YACC uses CFGs to construct a bottom-up parser. Learn about compiling with YACC, printing in C, Lex file format, YACC file format, and how to run and test programs using Lex and YACC.
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Intro to Lex and YACC Intro to Lex and YACC CSCI 432 Computer Science Theory CSCI 432 Computer Science Theory
Overview Overview Lex uses regular expressions to build a lexical analyzer. The function is named yylex() YACC uses CFGs to build a bottom-up parser. YACC = Yet Another Compiler Compiler The function yyparse() builds a parse tree. yyparse() calls yylex() over and over. When yyparse() finds a syntax error, it calls yyerror() then dies.
Compiling Compiling yacc -d project.yacc creates two files: y.tab.c and y.tab.h lex project.lex creates the file lex.yy.c cc y.tab.c lex.yy.c -ll -ly combines lex and yacc to create executable a.out
printing in C printing in C #include <stdio.h> int cnt = 0; char var[10] = "hello"; printf("hi mom\n"); printf("int variable cnt contains %d\n", cnt); printf("string var contains %s\n", var);
Lex File Format Lex File Format %{ //any C code you need for yylex() #include <stdio.h> #include <string.h> #include "y.tab.h" %} declarations %% regular_expressions actions %% driver code, eg main()
YACC File Format YACC File Format %{ #include <stdio.h> #include <string.h> probably some global variables %} %token list of tokens (all your terminals) %% grammar rules actions %% main() yyerror()
%{ #include <stdio.h> %} %% "-" printf ("MINUS\n"); "+" printf ("PLUS\n"); [0-9]+ printf ("NUM - %s\n", yytext); Lex file to find numbers, + and - \n ; . ; %% This yylex() is not called by yyparse(). int main() { while (yylex()); }
Compile and Run $ lex calc0.lex $ cc lex.yy.c -ll $ a.out 22 + 77 - 3 NUM - 22 PLUS NUM - 77 MINUS NUM - 3 99 + bob - joe * 13 NUM - 99 PLUS MINUS NUM - 13
%{ #include <stdio.h> #include <string.h> YACC file to create a very simple calculator extern int yylex(); void yyerror(char *); int num=0; int sum=0; part 1 of 3 %} %token NUM PLUS MINUS RETURN %%
YACC file to create a very simple calculator part 2 of 3 %% prog: stmts stmts: stmt | stmt stmts stmt: NUM { sum = num; } | PLUS NUM { sum = sum + num; } | MINUS NUM { sum = sum - num; } | RETURN { printf("--> %d\n",sum); } %%
%% int main() { yyparse(); } YACC file to create a very simple calculator void yyerror (char *msg) { printf("\n%s\n", msg); } part 3 of 3
%{ #include <stdio.h> #include <string.h> #include "y.tab.h" Lex file to create a very simple calculator extern int num; %} %% "-" return MINUS; "+" return PLUS; [0-9]+ { num=atoi(yytext); return NUM; } \n return RETURN; . ; %%
Sample Run of the Calculator $ a.out 99 --> 99 99 + 1 --> 100 + 50 --> 150 --> 150 10 + 10 + 10 + 10 - 5 --> 35 -35 --> 0 hi mom --> 0
Makefile Makefile lex.yy.c y.tab.h: calc.lex lex calc.lex y.tab.c: calc.yacc yacc -d calc.yacc calc: y.tab.c lex.yy.c cc y.tab.c lex.yy.c -ll -ly -o calc Tab, not spaces
