前期准备
为了写出C子集的词法分析器，首先应该了解C子集有哪些。
参考C 基本语法(菜鸟教程)先进行总结：
数字
需要考虑到十进制、十六进制、八进制、二进制、正负数、小数、科学计数法以及一些数字后缀：u，l，f等（注意浮点数不能加uU）。（应该没了吧？）
关键字：
auto 、break、case、char、const、continue、default、do、double、else、enum、extern、float、for、goto、if、int、long 、register、return、short、signed、sizeof、static、struct、switch、typedef、unsigned、union、void 、volatile、while。
标识符：
C 标识符是用来标识变量、函数，或任何其他用户自定义项目的名称。一个标识符以字母 A-Z 或 a-z 或下划线 _ 开始，后跟零个或多个字母、下划线和数字（0-9）。
运算符：
算数运算符：+、-、*、/、%、++、–
关系运算符：==、!=、>、<、>=、<=
逻辑运算符：&&、||、!
位运算符：&、|、^、～、<<、>>
赋值运算符：=、+=、-=、=、/=、%=、<<=、>>=、&=、^=、|=
杂项运算符我觉得单凭flex无法实现。
标点符号
三种括号：{ } [ ] ( ) 以及 : , ; . -> ” ‘
注释
两种注释：// /…*/
（应该没了吧？）
实现
脚本：
#!/bin/bash
flex -o C-lexical-analyzer.yy.c C-lexical-analyzer.l
echo "flex编译完成"
gcc -o C-lexical-analyzer C-lexical-analyzer.yy.c -lfl
echo "gcc编译完成"
rm C-lexical-analyzer.yy.c
echo "正在执行：(Ctrl+D可结束输入)"
./C-lexical-analyzer
rm C-lexical-analyzer
flex 代码：
/*
*file: C-lexical-analyzer.l
*auther: jin1ming
*system: manjaro
*/
%option yylineno
%{
#include<stdio.h>
extern int yylineno;
%}
/*数字定义*/
/*科学计数表示*/
science {decimal}(\.[0-9]+)?([Ee][-+]?[0-9]+)?
/*十进制*/
decimal ([-+])?(0|[1-9][0-9]*)
/*十六进制*/
hexadecimal 0[xX][a-fA-F0-9]+
/*二进制*/
binary 0[bB][01]+
/*八进制*/
octal 0[0-7]+
/*总表示*/
number ({hexadecimal}|{binary}|{science}|{octal})(([uU]?[Ll]?)|([Ll]?[Uu]?)|([fF]?))
/*注意浮点数总是有符号，不需要Uu后缀，所以在接下来单做一个浮点数异常处理*/
/*数字异常处理*/
floatexcption {decimal}\.[0-9]+([Ee]?[-+]?[0-9]+)?[Uu]
excption [0-9][0-9a-zA-Z\.]+
/*关键字*/
AUTO auto
BREAK break
CASE case
CHAR char
CONST const
CONTINUE continue
DEFAULT default
DO do
DOUBLE double
ELSE else
ENUM enum
EXTERN extern
FLOAT float
FOR for
GOTO goto
IF if
INT int
LONG long
REGISTER register
RETURN return
SHORT short
SIGNED signed
SIZEOF sizeof
STATIC static
STRUCT struct
SWITCH switch
TYPEDEF typedef
UNSIGNED unsigned
UNION union
VOID void
VOLATILE volatile
WHILE while
/*标识符定义*/
identifier [a-z_A-Z][a-z_A-Z0-9]*
/*其它字符*/
comment (\/\/.*)|(\/\*(.|\n)*\/)
whitespace [ \t\n\r\f\v]+
errno .
/*运算符*/
/*算术运算符*/
ADD \+
SUB \-
MUL \*
QUO \/
REM %
INC \+\+
DEC \-\-
/*赋值运算符*/
ASSIGN =
ADD_ASSIGN \+=
SUB_ASSIGN \-=
MUL_ASSIGN \*=
QUO_ASSIGN \/=
REM_ASSIGN %=
AND_ASSIGN \&=
OR_ASSIGN \|=
XOR_ASSIGN \^=
SHL_ASSIGN <<=
SHR_ASSIGN >>=
AND_NOT_ASSIGN ~=
/*位运算符*/
AND &
OR \|
XOR \^
SHL <<
SHR >>
AND_NOT ~
/*逻辑运算符*/
LAND &&
LOR \|\|
NOT \!
/*关系运算符*/
EQL ==
LSS <
GTR >
NEQ !=
LEQ <=
GEQ >=
/*标点符号*/
LPAREN \(
LBRACK \[
LBRACE \{
COMMA ,
PERIOD \.
RPAREN \)
RBRACK \]
RBRACE \}
SEMICOLON ;
COLON :
POT \->
DQUA \"
SQUA \'
%%
/*关键字*/
{AUTO} {printf("Key Word: %s\n",yytext);}
{BREAK} {printf("Key Word: %s\n",yytext);}
{CASE} {printf("Key Word: %s\n",yytext);}
{CHAR} {printf("Key Word: %s\n",yytext);}
{CONST} {printf("Key Word: %s\n",yytext);}
{CONTINUE} {printf("Key Word: %s\n",yytext);}
{DEFAULT} {printf("Key Word: %s\n",yytext);}
{DO} {printf("Key Word: %s\n",yytext);}
{DOUBLE} {printf("Key Word: %s\n",yytext);}
{ELSE} {printf("Key Word: %s\n",yytext);}
{ENUM} {printf("Key Word: %s\n",yytext);}
{EXTERN} {printf("Key Word: %s\n",yytext);}
{FLOAT} {printf("Key Word: %s\n",yytext);}
{FOR} {printf("Key Word: %s\n",yytext);}
{GOTO} {printf("Key Word: %s\n",yytext);}
{IF} {printf("Key Word: %s\n",yytext);}
{INT} {printf("Key Word: %s\n",yytext);}
{LONG} {printf("Key Word: %s\n",yytext);}
{REGISTER} {printf("Key Word: %s\n",yytext);}
{RETURN} {printf("Key Word: %s\n",yytext);}
{SHORT} {printf("Key Word: %s\n",yytext);}
{SIGNED} {printf("Key Word: %s\n",yytext);}
{SIZEOF} {printf("Key Word: %s\n",yytext);}
{STATIC} {printf("Key Word: %s\n",yytext);}
{STRUCT} {printf("Key Word: %s\n",yytext);}
{SWITCH} {printf("Key Word: %s\n",yytext);}
{TYPEDEF} {printf("Key Word: %s\n",yytext);}
{UNSIGNED} {printf("Key Word: %s\n",yytext);}
{UNION} {printf("Key Word: %s\n",yytext);}
{VOID} {printf("Key Word: %s\n",yytext);}
{VOLATILE} {printf("Key Word: %s\n",yytext);}
{WHILE} {printf("Key Word: %s\n",yytext);}
/*提前处理浮点数+uU的异常*/
{floatexcption} {printf("Float Execption: %s\n",yytext);}
/*数字表示*/
{number} {printf("Number: %s\n",yytext);}
/*异常数字处理*/
{excption} {printf("Number Execption: %s\n",yytext);}
/*跳过空白和注释*/
{whitespace} {}
{comment} {printf("This is a commit.\n");}
/*运算符*/
/*算术运算符*/
{ADD} {printf("Operator: %s\n",yytext);}
{SUB} {printf("Operator: %s\n",yytext);}
{MUL} {printf("Operator: %s\n",yytext);}
{QUO} {printf("Operator: %s\n",yytext);}
{REM} {printf("Operator: %s\n",yytext);}
{INC} {printf("Operator: %s\n",yytext);}
{DEC} {printf("Operator: %s\n",yytext);}
/*逻辑运算符*/
{LAND} {printf("Operator: %s\n",yytext);}
{LOR} {printf("Operator: %s\n",yytext);}
{NOT} {printf("Operator: %s\n",yytext);}
/*赋值运算符*/
{ASSIGN} {printf("Operator: %s\n",yytext);}
{ADD_ASSIGN} {printf("Operator: %s\n",yytext);}
{SUB_ASSIGN} {printf("Operator: %s\n",yytext);}
{MUL_ASSIGN} {printf("Operator: %s\n",yytext);}
{QUO_ASSIGN} {printf("Operator: %s\n",yytext);}
{REM_ASSIGN} {printf("Operator: %s\n",yytext);}
{AND_ASSIGN} {printf("Operator: %s\n",yytext);}
{OR_ASSIGN} {printf("Operator: %s\n",yytext);}
{XOR_ASSIGN} {printf("Operator: %s\n",yytext);}
{SHL_ASSIGN} {printf("Operator: %s\n",yytext);}
{SHR_ASSIGN} {printf("Operator: %s\n",yytext);}
{AND_NOT_ASSIGN} {printf("Operator: %s\n",yytext);}
/*位运算符*/
{AND} {printf("Operator: %s\n",yytext);}
{OR} {printf("Operator: %s\n",yytext);}
{XOR} {printf("Operator: %s\n",yytext);}
{SHL} {printf("Operator: %s\n",yytext);}
{SHR} {printf("Operator: %s\n",yytext);}
{AND_NOT} {printf("Operator: %s\n",yytext);}
/*关系运算符*/
{EQL} {printf("Operator: %s\n",yytext);}
{LSS} {printf("Operator: %s\n",yytext);}
{GTR} {printf("Operator: %s\n",yytext);}
{NEQ} {printf("Operator: %s\n",yytext);}
{LEQ} {printf("Operator: %s\n",yytext);}
{GEQ} {printf("Operator: %s\n",yytext);}
/*标点符号*/
{LPAREN} {printf("Punctuation: %s\n",yytext);}
{LBRACK} {printf("Punctuation: %s\n",yytext);}
{LBRACE} {printf("Punctuation: %s\n",yytext);}
{COMMA} {printf("Punctuation: %s\n",yytext);}
{PERIOD} {printf("Punctuation: %s\n",yytext);}
{RPAREN} {printf("Punctuation: %s\n",yytext);}
{RBRACK} {printf("Punctuation: %s\n",yytext);}
{RBRACE} {printf("Punctuation: %s\n",yytext);}
{SEMICOLON} {printf("Punctuation: %s\n",yytext);}
{COLON} {printf("Punctuation: %s\n",yytext);}
{POT} {printf("Punctuation: %s\n",yytext);}
{DQUA} {printf("Punctuation: %s\n",yytext);}
{SQUA} {printf("Punctuation: %s\n",yytext);}
{identifier} {printf("ID: %s\n",yytext);}
{errno} {printf("On line %d,mystery character: %s\n",yylineno,yytext);}
%%
int main(int argc,char **argv)
{
yylineno = 1;
yylex();
return 0;
}
int yywarp(){
return 1;
}
测试效果：
$ ./start.sh #编译脚本
编译完成，请手动执行C-lexical-analyzer
[jin1ming@ML C-Lex]$ ./C-lexical-analyzer
78.987e76f
Number: 78.987e76f
@
On line 2,mystery character: @
adsda
ID: adsda
srg090
ID: srg090
_12
ID: _12
!@#$#
Operator: !
121qwqer
Number Execption: 121qwqer
1.2.
Number Execption: 1.2.
1.2uf
Number Execption: 1.2uf
==
Operator: ==
-
Operator: -
=
Operator: =
^
Operator: ^
!
Operator: !
>>
Operator: >>
,.!
Punctuation: ,
Punctuation: .
Operator: !
