![Lex program structure
β¦ definitions β¦
%%
β¦ rules β¦
%%
β¦ subroutines β¦
%{
#include <stdio.h>
#include "y.tab.h"
int c;
extern int yylval;
%}
%%
" " ;
[a-z] { c = yytext[0]; yylval = c - 'a';
return(LETTER); }
[0-9]* { yylval = atoi(yytext);
return(NUMBER); }
[^a-z0-9b] { c = yytext[0]; return(c); }](https://image.slidesharecdn.com/lexandyacc-250119063824-48f944a6/85/Compiler-Design-Brief-in-Computer-Application-9-320.jpg)
![Pattern Matching and Action
[a-z] { c = yytext[0]; yylval = c - 'a';
return(LETTER); }
[0-9]* { yylval = atoi(yytext);
return(NUMBER); }
Match a character
in the a-z range
Match a positive integer
(sequence of 0-9 digits)
Place the offset c β βaβ
In the stack
Place the integer value
In the stack
Buffer](https://image.slidesharecdn.com/lexandyacc-250119063824-48f944a6/85/Compiler-Design-Brief-in-Computer-Application-10-320.jpg)
![Yacc Program Structure
%{
#include <stdio.h>
int regs[26];
int base;
%}
%token NUMBER LETTER
%left '+' '-β
%left '*' '/β
%%
list: | list stat 'n' |list error 'n' {yyerrok;} ;
stat: expr {printf("%dn",$1);}
| LETTER '=' expr {regs[$1] = $3;};
expr:
'(' expr ')' {$$ = $2;} |
expr '+' expr {$$ = $1 + $3;} |
LETTER {$$ = regs[$1];}
%%
main(){return(yyparse());}
yyerror(CHAR *s){fprintf(stderr, "%sn",s);}
yywrap(){ return(1);}
β¦ definitions β¦
%%
β¦ rules β¦
%%
β¦ subroutines β¦](https://image.slidesharecdn.com/lexandyacc-250119063824-48f944a6/85/Compiler-Design-Brief-in-Computer-Application-12-320.jpg)
![Rule Reduction and Action
stat: expr {printf("%dn",$1);}
| LETTER '=' expr {regs[$1] = $3;};
expr:
expr '+' expr {$$ = $1 + $3;} |
LETTER {$$ = regs[$1];}
Grammar rule Action
βorβ operator:
For multiple RHS](https://image.slidesharecdn.com/lexandyacc-250119063824-48f944a6/85/Compiler-Design-Brief-in-Computer-Application-13-320.jpg)
Compiler Design Brief in Computer Application
- 1. Lex and Yacc COP - 3402
- 2. General Compiler Infra-structure Scanner (tokenizer) Parser Semantic Routines Analysis/ Transformations/ optimizations Code Generator Program source (stream of characters) Tokens Syntactic Structure IR: Intermediate Representation (1) Assembly code IR: Intermediate Representation (2) Symbol and Attribute Tables Lex Yacc
- 3. Lex & Yacc β’ Lex β generates C code for the lexical analyzer (scanner) β Token patterns specified by regular expressions β’ Yacc β generates C code for a LR(1) syntax analyzer (parser) β BNF rules for the grammar
- 4. Lex ο lex is a program (generator) that generates lexical analyzers, (widely used on Unix). ο It is mostly used with Yacc parser generator. ο Written by Eric Schmidt and Mike Lesk. ο It reads the input stream (specifying the lexical analyzer ) and outputs source code implementing the lexical analyzer in the C programming language. ο Lex will read patterns (regular expressions); then produces C code for a lexical analyzer that scans for identifiers.
- 5. Lex predefined variables and functions
- 6. Lex β Pattern Matching Primitives
- 7. Lex β Pattern Matching Examples
- 8. Example: Simple Calculator β’ Computes the basic arithmetic operations β’ Allows declaration of variables β’ Enough to illustrate the basic structure of Lex and Yacc programs
- 9. Lex program structure β¦ definitions β¦ %% β¦ rules β¦ %% β¦ subroutines β¦ %{ #include <stdio.h> #include "y.tab.h" int c; extern int yylval; %} %% " " ; [a-z] { c = yytext[0]; yylval = c - 'a'; return(LETTER); } [0-9]* { yylval = atoi(yytext); return(NUMBER); } [^a-z0-9b] { c = yytext[0]; return(c); }
- 10. Pattern Matching and Action [a-z] { c = yytext[0]; yylval = c - 'a'; return(LETTER); } [0-9]* { yylval = atoi(yytext); return(NUMBER); } Match a character in the a-z range Match a positive integer (sequence of 0-9 digits) Place the offset c β βaβ In the stack Place the integer value In the stack Buffer
- 11. Yacc β’ Grammars described by rules using a variant of the Backus Naur Form (BNF) β Context-free grammars β’ LALR(1) parse table is generated automatically based on the rules β’ Actions are added to the rules and executed after each reduction
- 12. Yacc Program Structure %{ #include <stdio.h> int regs[26]; int base; %} %token NUMBER LETTER %left '+' '-β %left '*' '/β %% list: | list stat 'n' |list error 'n' {yyerrok;} ; stat: expr {printf("%dn",$1);} | LETTER '=' expr {regs[$1] = $3;}; expr: '(' expr ')' {$$ = $2;} | expr '+' expr {$$ = $1 + $3;} | LETTER {$$ = regs[$1];} %% main(){return(yyparse());} yyerror(CHAR *s){fprintf(stderr, "%sn",s);} yywrap(){ return(1);} β¦ definitions β¦ %% β¦ rules β¦ %% β¦ subroutines β¦
- 13. Rule Reduction and Action stat: expr {printf("%dn",$1);} | LETTER '=' expr {regs[$1] = $3;}; expr: expr '+' expr {$$ = $1 + $3;} | LETTER {$$ = regs[$1];} Grammar rule Action βorβ operator: For multiple RHS
- 14. Further readingβ¦ ο βA Compact Guide to Lex & Yaccβ, Thomas Niemann (recommended); ο βLex & Yaccβ, Doug Brown (OβReily); ο Lots of resources on the web β’ Check our website for some suggestions
- 15. Conclusions β’ Yacc and Lex are very helpful for building the compiler front-end β’ A lot of time is saved when compared to hand-implementation of parser and scanner β’ They both work as a mixture of βrulesβ and βC codeβ β’ C code is generated and is merged with the rest of the compiler code