詞法分析器就是通過掃描一段程序判斷是否是關鍵字、標識符、常數、分界符、運算符。一般分為一符一種和經典五中；

這裏我用的是經典五中，此詞法分析器是用c++編寫的；

/*
保留字|關鍵字：1
操作符|運算符：2
分界符：3
標識符：4
常數：5
無識別：6
*/

主要代碼為：

#include<iostream>
using namespace std;
#define MAX 10
/*
	保留字|關鍵字：1
	操作符|運算符：2
	分界符：3
	標識符：4
	常數：5
	無識別：6
*/
char ch = ‘ ‘;
char* keyWord[10] = {"void","main","break","include","begin","end","if","else","while","switch"};
char token[20];//定義獲取的字符
//判斷是否是關鍵字
bool isKey(char * token)
{
	for(int i = 0;i < MAX;i++)
	{
		if(strcmp(token,keyWord[i]) == 0)
			return true;
	}
	return false;
}
//判斷是否是字母
bool isLetter(char letter)
{
	if((letter >= ‘a‘ && letter <= ‘z‘)||(letter >= ‘A‘ && letter <= ‘Z‘))
		return true;
	else
		return false;
}
//判斷是否是數字
bool isDigit(char digit)
{
	if(digit >= ‘0‘ && digit <= ‘9‘)
		return true;
	else
		return false;
}
//詞法分析
void analyze(FILE *fpin)
{
	
	while((ch = fgetc(fpin)) != EOF){
		if(ch == ‘ ‘||ch == ‘\t‘||ch == ‘\n‘){}
		else if(isLetter(ch)){
			char token[20]={‘\0‘};
			int i=0;
			while(isLetter(ch)||isDigit(ch)){	
				token[i] = ch;
				i++;
				ch = fgetc(fpin);
			}
			//回退一個指針
			fseek(fpin,-1L,SEEK_CUR);
			if(isKey(token)){
				//關鍵字
				cout<<token<<"\t1"<<"\t關鍵字"<<endl;
			}
			else{
				//標識符
				cout<<token<<"\t4"<<"\t標識符"<<endl;		
			}
		}
		else if(isDigit(ch)||(ch == ‘.‘))
		{
			int i=0;
			char token[20]={‘\0‘};
			while(isDigit(ch)||(ch == ‘.‘&&isDigit(fgetc(fpin))))
			{
				if(ch == ‘.‘)fseek(fpin,-1L,SEEK_CUR);
				token[i] = ch;
				i++;
				ch = fgetc(fpin);
			}
			fseek(fpin,-1L,SEEK_CUR);
			//屬於無符號常數
			cout<<token<<"\t5"<<"\t常數"<<endl;
		}
		else switch(ch){
			//運算符
			case ‘+‘:{
						ch = fgetc(fpin);
						if(ch == ‘+‘)cout<<"++"<<"\t2"<<"\t運算符"<<endl;
						else {
							cout<<"+"<<"\t2"<<"\t運算符"<<endl;
							fseek(fpin,-1L,SEEK_CUR);
						}
					 }break;
			case ‘-‘:{
						ch = fgetc(fpin);
						if(ch == ‘-‘)cout<<"--"<<"\t2"<<"\t運算符"<<endl;
						else {
							cout<<"-"<<"\t2"<<"\t運算符"<<endl;
							fseek(fpin,-1L,SEEK_CUR);
						}
					 }break;
			case ‘*‘:cout<<ch<<"\t2"<<"\t運算符"<<endl;break;
			case ‘/‘:cout<<ch<<"\t2"<<"\t運算符"<<endl;break;
			//分界符
			case ‘(‘:cout<<ch<<"\t3"<<"\t分界符"<<endl;break;
			case ‘)‘:cout<<ch<<"\t3"<<"\t分界符"<<endl;break;
			case ‘[‘:cout<<ch<<"\t3"<<"\t分界符"<<endl;break;
			case ‘]‘:cout<<ch<<"\t3"<<"\t分界符"<<endl;break;
			case ‘;‘:cout<<ch<<"\t3"<<"\t分界符"<<endl;break;
			case ‘{‘:cout<<ch<<"\t3"<<"\t分界符"<<endl;break;
			case ‘}‘:cout<<ch<<"\t3"<<"\t分界符"<<endl;break;
			//運算符
			case ‘=‘:{
						ch = fgetc(fpin);
						if(ch == ‘=‘)cout<<"=="<<"\t2"<<"\t運算符"<<endl;
						else {
							cout<<"="<<"\t2"<<"\t運算符"<<endl;
							fseek(fpin,-1L,SEEK_CUR);
						}
					 }break;
			case ‘:‘:{
						ch = fgetc(fpin);
						if(ch == ‘=‘)cout<<":="<<"\t2"<<"\t運算符"<<endl;
						else {
							cout<<":"<<"\t2"<<"\t運算符"<<endl;
							fseek(fpin,-1L,SEEK_CUR);
						}
					 }break;
			case ‘>‘:{
						ch = fgetc(fpin);
						if(ch == ‘=‘)cout<<">="<<"\t2"<<"\t運算符"<<endl;
						else {
							cout<<">"<<"\t2"<<"\t運算符"<<endl;
							fseek(fpin,-1L,SEEK_CUR);
						}
					 }break;
			case ‘<‘:{
						ch = fgetc(fpin);
						if(ch == ‘=‘)cout<<"<="<<"\t2"<<"\t運算符"<<endl;
						else {
							cout<<"<"<<"\t2"<<"\t運算符"<<endl;
							fseek(fpin,-1L,SEEK_CUR);
						}
					 }break;
			//無識別
			default: cout<<ch<<"\t6"<<"\t無識別符"<<endl;
		}
	}
}
int main(){
	char input[30];
	FILE *fpin;
	cout<<"請輸入源文件名：\n"<<endl;
	for(;;){
		cin>>input;
		if((fpin = fopen(input,"r")) != NULL)
			break;
		else 
			cout<<"路徑輸入錯誤"<<endl;
	}
	cout<<"****************詞法分析結果********************"<<endl;
	analyze(fpin);
	fclose(fpin);
}

　　運行結果：

c++詞法分析器