词法分析是指将我们编写的文本代码流解析为一个一个的记号，分析得到的记号以供后续语法分析使用。词法分析的工作是低级别的分析:将字符或者字符序列转化成记号.

#include<stdio.h>
#include<string.h>
#define t 10
#define num2 11
void analyse(FILE *fp,char ch);
int main()
{
FILE *fp;
char ch;
if((fp=fopen("demo.txt","w"))==NULL)
{
printf("Fileure to open demo.txt!\n");
}
printf("请输入源程序(以$结束):");
ch=getchar();
while(ch!=‘$‘)
{
fputc(ch,fp);
ch=getchar();
}
fclose(fp);
if((fp=fopen("demo.txt","r"))==NULL)
{
printf("Fileure to open demo.txt!\n");
}
ch=fgetc(fp);
analyse(fp,ch);
}
void analyse(FILE *fp,char ch)
{
int m,j,i=0,n,flag=0;
FILE *fp1,*fp2;
char s;
char gjz[34][20]={"short","if","do","while","end","auto","double","int","struct","break"," "," ","else","long","switch","case","enum","register","typedef","char","extern","union","const","float","unsigned","continue","signed","for","default",
"void","goto","sizeof","volatile","static"};
char num[5];
char bsf[10];
char alp[15];
while(!feof(fp))
{
m=0,j=0;
i=0;
if(ch==‘+‘)
printf("%c\t%d\n",ch,35);
else if(ch==‘-‘)
printf("%c\t%d\n",ch,36);
else if(ch==‘*‘)
printf("%c\t%d\n",ch,37);
else if(ch==‘/‘)
printf("%c\t%d\n",ch,38);
else if(ch==‘=‘)
printf("%c\t%d\n",ch,39);
else if(ch==‘#‘)
printf("%c\t%d\n",ch,40);
else if(ch==‘>‘)
{
s=fgetc(fp);
if(s==‘=‘)
printf("%c=\t%d\n",ch,43);
else
{
printf("%c\t%d\n",ch,41);
}
analyse(fp,s);
}
else if(ch==‘<‘)
{
s=fgetc(fp);
if(s==‘=‘)
printf("%c=\t%d\n",ch,44);
else
{
printf("%c\t%d\n",ch,42);
}
analyse(fp,s);
}
else if(ch==‘.‘)
printf("%c\t%d\n",ch,53);
else if(ch==‘:‘)
{ s=fgetc(fp);
if(s==‘=‘)
printf("%c=\t%d\n",ch,45);
else
{
printf("%c\t%d\n",ch,46);
}
analyse(fp,s);
}
else if(ch==‘(‘)
printf("%c\t%d\n",ch,47);
else if(ch==‘)‘)
printf("%c\t%d\n",ch,48);
else if(ch==‘;‘)
printf("%c\t%d\n",ch,54);
else if(ch==‘,‘)
printf("%c\t%d\n",ch,49);
else if(ch==‘{‘)
printf("%c\t%d\n",ch,50);
else if(ch==‘}‘)
printf("%c\t%d\n",ch,51);
else if(ch==‘"‘)
{
printf("%c\t%d\n",ch,52);
}
else if(ch>=‘a‘&&ch<=‘z‘||ch>=‘A‘&&ch<=‘Z‘)
{
if((fp1=fopen("data.txt","w"))==NULL)
{
printf("Fileure to open data.txt!\n");
}
do
{
fputc(ch,fp1);
i++;
ch=fgetc(fp);
}while(ch>=‘a‘&&ch<=‘z‘||ch>=‘A‘&&ch<=‘Z‘||ch>=‘0‘&&ch<=‘9‘);
fclose(fp1);
if((fp1=fopen("data.txt","r"))==NULL)
{
printf("Fileure to open data.txt!\n");
}
fgets(alp,i+1,fp1);
for(j=0;j<34;j++)
{
if(j==10)
j=j+2;
if(strcmp(alp,gjz[j])==0)
{
printf("%s\t%d\n",gjz[j],j);
flag=1;
break;
}
}
if(flag==0)
printf("%s\t%d\n",alp,t);
fclose(fp1);
analyse(fp,ch);
}
else if(ch>=‘0‘&&ch<=‘9‘)
{
if((fp2=fopen("data2.txt","w"))==NULL)
{
printf("Fileure to open data2.txt!\n");
}
do
{
fputc(ch,fp2);
m++;
ch=fgetc(fp);
} while(ch>=‘0‘&&ch<=‘9‘);
fclose(fp2);
if((fp2=fopen("data2.txt","r"))==NULL)
{
printf("Fileure to open data2.txt!\n");
}
fgets(num,m+1,fp2);
printf("%s\t%d\n",num,num2);
fclose(fp2);
analyse(fp,ch);
}
else
printf("\r");
ch=fgetc(fp);
}
}

<字母>：S-〉A|AA

　　　　　A->{a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,w,y,

　　　　　　　　A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,S,Y,Z}
<数字>：S->A|AA|A0|A.

　　　　　A->{1,2,3,4,5,6,7,8,9}
<整数常数> ：S->A|AA|A0

　　　　  　   A->{1,2,3,4,5,6,7,8,9}

<标识符>：S->A|A_|AB

　　　　　　A->{a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,w,y,

　　　　　　　　A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,S,Y,Z}

　　　　　　B->{0,1,2,3,4,5,6,7,8,9}

<关键字>：S->{

short","if","do","while","end","auto","double","int","struct","break","else","long","switch","case","enum","register","typedef","char","extern","union","const","float","unsigned","continue","signed","for","default"，"void","goto","sizeof","volatile","static"}

<运算符>：S->{+,-，*,/,>,<,==，<=,>=}
<界符>； S->{; , ( , ) , ,{,},‘‘ }

词法分析