编撰最简单的SQL解析程序（原理演示）

如何写一个SQL解析器呢？这里先抛出第一步：单词切分。下面举个简单例子：

select.l 文件：

%{/* nothing */  enum{    BEGIN_OF_INPUT = 0,    SELECT,    COLUMN,    FROM,    TABLE,    SEP,    END_OF_INPUT  };  int state = BEGIN_OF_INPUT;%}%%\n {  if (SEP == state)  {    printf("=========   Cong!!! found a valid sql   ==========\n");  }  else if (BEGIN_OF_INPUT < state)  {    printf("=========   :-(  not a valid sql   ==========\n");  }  state = BEGIN_OF_INPUT;  printf("please input a sql and press Enter\n");}SELECT {  if (state == (SELECT - 1))    state++;  printf("select state=%d\n", state);}FROM {    if (state == (FROM - 1))          state++;  printf("from state=%d\n", state);}[a-zA-Z_]+ {  if (state == (COLUMN - 1) || state == (TABLE - 1))    state++;  printf("any word state=%d\n", state);}; {  if (state == (SEP - 1))  {    state++;  }  printf("seperator. state=%d\n", state);}. ; /* ignore others */%%main(){  yylex();}

lex select.l     gcc lex.yy.c  -ll

./a.out      

注意：

写lex文件规则的时候，严格的规则必须写在松散的规则前面。下面是一个错误的例子：

[a-zA-Z_]+ {  if (state == (COLUMN - 1) || state == (TABLE - 1))    state++;  printf("any word state=%d\n", state);}FROM {    if (state == (FROM - 1))          state++;  printf("from state=%d\n", state);}

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5年前写过的关于lex yacc的文章：

VC与YACC、LEX集成 

一个简单的C语言词法分析与语法分析器【原】