Post your code.

Code is

    #include 
    #include 
    #include 
    #include 
    int isValid(char[],char[]);//function declaration

    struct student //structure defination
    {
    char name[25];
    char course[25];
    char rollNum[20];    
    };

    //isvalid function for checking valid rollnumber pattren and name

    int isValid(char str[],char str1[]){
    int i=0,j=0,flag=0,flag1=0;

    for(i=0;i= 'A' && str[i] <= 'Z') ||(str[i] >= 'a' && str[i] <= 'z'))
           flag=0;
           else
           if((str[i]>='0'&&str[i]<='9')||((str[i]>='A'&&str[i]<='Z')||(str[i]>='a'&&str[i]<='z'))) 
           flag=1;  

          }  /*checking for valid name only characters*/
          for(j=0;j='A'&&str1[j]<='Z')||(str1[j]>='a'&&str1[j]<='z'))
           flag1=1;
           else
           flag1=0;  
           }
    return (flag && flag1);
    }

       //main function
    int main()
    {
    int choice,rCheck=0,k=0,flag;
    FILE *fp;
    struct student s;
    char loop,search[20],rollNum1[25];


    //open file in append mode
    fp=fopen("Student.dat","a+");

    if(fp==NULL)
    {
    printf("FILE CAN'T OPEN.\n");
    exit(1);
    }
    printf("\n\tOUT PUT 2\n");

    //menu
    do
    {flag=1;
    printf("\n\t1.Add Record\n");
    printf("\t2.Display Records\n");
    printf("\t3.EXIT\n");
    printf("\tEnter your choice...\n");
    scanf("%d",&choice);


    switch(choice)
    {

    //Add Record in file
    int k;
    case 1:printf("\n\tEnter Roll number (Ex.1234 or a123)\n");

//Here i want to change case
//...........................................

           scanf("%s",s.rollNum);

//.....................................
          fseek(fp,0,SEEK_SET);

           /*searching rollnumber in file for avoiding duplicate rollnum*/
           while(fscanf(fp,"%s",search)!=EOF){                                      
           if(strcmp(search,s.rollNum)==0){
           printf("This roll number already exist!!!!!");
           flag=0;break;}}

            /* rollnumber not exist then add data*/ 
           if(flag!=0){ 
           printf("\n\tEnter name and subject\n");
           scanf("\n\t%s %s",s.name,s.course);
           rCheck=isValid(s.rollNum,s.name);
           if(rCheck==1)        
           fprintf(fp,"%s %s %s \n" ,s.rollNum,s.name,s.course);
           else
           printf("\n\n!!!!Enter correct rollnumber and Name!!!!!");
           }
           break;

    //Display records from begining

   case 2:fseek(fp,0,SEEK_SET);
          printf("\n\tROLLNUMBER\tNAME\t\tCOURSE\n");
          while(fscanf(fp,"%s %s %s",s.rollNum,s.name,s.course)!=EOF)
          printf("\n\n\t%s \t\t%5s \t\t%s \n",s.rollNum,s.name,s.course);
          break;

    //exit
    case 3:exit(1);
    /*default*/
    default:printf("\n\n\t\a    !!!!!! WARNING !!!!! \a\n\t");
            printf("\n\t\a!!!!!! Please enter valid choice !!!!! \n\t"); /*default case*/
            system("pause");
            exit(1);
    }

     printf("\n\n\t Do you want to continue (Y/N) : ");
     loop=getche();

    }while(loop=='y'||loop=='Y');


    //close file
    fclose(fp);

    }

Can anybody help me in solving this question?