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《Flex & Bison》阅读笔记
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Reading
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2018-10-04 3127字

最近在看 《Flex & Bison》 这本书, 针对书中的例子进行解读和笔记, 以消化每一个知识细节, 所以本文会持续更新, 直到《Flex & Bison》读完.

因为本文只是针对源码程序进行简单的笔记备忘, 不会完全讲解, 所以为了弄懂每行代码, 建议阅读《Flex & Bison》原文.

运行环境:

单词统计程序

wc.l 源代码:

%{
    int chars = 0;
    int words = 0;
    int lines = 0;
%}

%%
[^ \t\n\r\f\v]+ { words++; chars += strlen(yytext); }
\n              { chars++; lines++; }
.               { chars++; }
%%

int main(int argc, char **argv) {
    yylex();
    printf("%8d%8d%8d\n", lines, words, chars);
}

编译命令: compile wc.l

源码备注:

英美式英语转换

convert.l 源代码:

%%
"colour"        { printf("color"); }
"flavour"       { printf("flavor"); }
"clever"        { printf("smart"); }
"conservative"  { printf("libreal"); }
.               { printf("%s", yytext); }
%%

int main(int argc, char **argv) {
    yylex();
}

编译命令: compile convert.l

源码备注:

计算器

calculator.y

%{
#define YYSTYPE double
#include<stdio.h>
#include<math.h>
int yylex();
void yyerror(char *s);
%}

%token NUMBER
%token ADD SUB MUL DIV ABS
%token EOL
%token OP CP
%token POW SQRT

%%
calclist:
| calclist exp EOL { printf("= %f\n>>> ", $2); }
| calclist EOL { printf(">>> "); } /* blank line or a comment */
;

exp: factor { $$ = $1; }
| exp ADD factor { $$ = $1 + $3; }
| exp SUB factor { $$ = $1 - $3; }
;

factor: another_factor { $$ =$1; }
| factor MUL another_factor { $$ = $1 * $3; }
| factor DIV another_factor { $$ = $1 / $3; }
;

another_factor: term { $$ = $1; }
| SUB another_factor { $$ = -$2; }
| another_factor POW another_factor { $$ = pow($1,$3); }
;

term: NUMBER { $$ = $1; }
| ABS exp ABS { $$ = fabs($2); }
| OP exp CP { $$ = $2; }
| SQRT OP exp CP { $$ = sqrt($3); }
;
%%

int main(int argc, char **argv) {
    printf(">>> ");
    yyparse();
}

void yyerror(char *s) {
    fprintf(stderr,"error : %s\n",s);
}

calculator.l

%{
#define YYSTYPE double
#include "calculator.tab.h"
#include <stdlib.h>
YYSTYPE yylval;
%}

%%
"+"                             { return ADD; }
"-"                             { return SUB; }
"*"                             { return MUL; }
"/"                             { return DIV; }
"|"                             { return ABS; }
"("                             { return OP; }
")"                             { return CP; }
([0-9]*\.?[0-9]+|[0-9]+\.)      { yylval = atof(yytext); return NUMBER;}
"sqrt"                          { return SQRT; }
"**"                            { return POW; }
\n                              { return EOL; }
[ \t]                           {}
"//".*                          {}
%%

编译命令: compile calculator

源码备注:

Flex 的正则表达式

Flex 有几个正则表达式和传统的正则表达式规则还是有点区别:

统计文件中的单词数

wc-file.l 源代码:

%option noyywrap

%{
    int chars = 0;
    int words = 0;
    int lines = 0;

    int totchars = 0;
    int totwords = 0;
    int totlines = 0;
%}

%%
[a-zA-Z]+       { words++; chars += strlen(yytext); }
\n              { chars++; lines++; }
.               { chars++; }
%%

int main(int argc, char **argv) {
    printf("%8s%8s%8s%8s\n", "lines", "words", "chars", "file");

    if (argc < 2) {
        yylex();
        printf("%8d%8d%8d\n", lines, words, chars);
        return 0;
    }

    for (int i = 1; i < argc; i++) {
        FILE *f = fopen(argv[i], "r");

        if (!f) {
            perror(argv[i]);
            return 1;
        }

        yyrestart(f);
        yylex();
        fclose(f);

        printf("%8d%8d%8d %s\n", lines, words, chars, argv[i]);

        totchars += chars; chars = 0;
        totwords += words; words = 0;
        totlines += lines; lines = 0;
    }

    if (argc > 1) {
        printf("%8d%8d%8d total\n", totlines, totwords, totchars);
    }

    return 0;
}

编译命令: compile wc-file.l

阅读笔记: