无意中在github上发现一个很有意思的项目。它利用flex实现了字符串的识别，利用bison实现了ast语法树的构建，最后直接利用ast进行计算和识别。
比如，它的lex文件是这么写的，
/***********************************/
/* File: cmm.l version 2.0 */
/* Flex version for CMM */
/* CMM Interpreter Construction */
/***********************************/
%option noyywrap
%{
#include "globals.h"
#include "util.h"
#include "scan.h"
#include "cmm.tab.h"
%}
digit [0-9]
int_num {digit}+
real_num {digit}+"."{digit}*
letter [a-zA-Z]
identifier {letter}+({letter}|{digit}|_)*({letter}+|{digit}+)|{letter}+
whitespace [ \t]*
%%
"if" { return IF;}
"else" { return ELSE;}
"while" { return WHILE;}
"int" { yylval.dataType = Int; return INT;}
"real" { yylval.dataType = Real; return REAL;}
"bool" { yylval.dataType = Bool; return BOOL;}
"read" { return READ;}
"write" { return WRITE;}
"main" { return MAIN;}
{int_num} { yylval.intval = atoi(yytext); return INT_VALUE;}
{real_num} { yylval.realval = atof(yytext); return REAL_VALUE;}
{identifier} { yylval.idName = copyString(yytext); return ID;}
{whitespace} {/*Do nothing.*/}
"+" { return PLUS;}
"-" { return SUB;}
"*" { return MUL;}
"/" { return DIV;}
"<" { yylval.binaryOperator = 1; return REL_OP;}
">" { yylval.binaryOperator = 2; return REL_OP;}
"<=" { yylval.binaryOperator = 3; return REL_OP;}
">=" { yylval.binaryOperator = 4; return REL_OP;}
"==" { yylval.binaryOperator = 5; return REL_OP;}
"<>" { yylval.binaryOperator = 6; return REL_OP;}
"&&" { yylval.binaryOperator = 7; return REL_OP;}
"||" { yylval.binaryOperator = 8; return REL_OP;}
"=" { return ASSIGN;}
"(" { return LPAREN;}
")" { return RPAREN;}
";" { return SEMI;}
"{" { return LBRACE;}
"}" { return RBRACE;}
"[" { return LBRACKET;}
"]" { return RBRACKET;}
"," { return COMMA;}
"//" { char c = input();
while(c != '\n')
{ if (c == EOF) break;
c = input();
}
lineno++;
}
"/*" { char c;
int flag = 1;
do
{ c = input();
entry1:
if (c == EOF) break;
if (c == '\n') lineno++;
if (c == '*')
{ c = input();
if (c == '/')
flag = 0;
else
goto entry1;
}
} while (flag);
}
\n {lineno++;}
. {yyerror("Mystery character %s\n", yytext);return ERROR;}
%%
/* 用于语法分析时初始化词法分析接口 */
void iniLexer(void)
{
static int firstTime = TRUE;
lineno = 0;
if (firstTime)
{ firstTime = FALSE;
lineno++;
yyin = source;
yyout = listing;
}
}
/* 词法分析器专用
TokenType getToken(void)
{ static int firstTime = TRUE;
TokenType currentToken;
if (firstTime)
{ firstTime = FALSE;
lineno++;
yyin = source;
yyout = listing;
}
currentToken = yylex();
strncpy(tokenString,yytext,MAXTOKENLEN);
if (TraceScan) {
fprintf(listing,"\t%d: ",lineno);
printToken(current