Write a program to check if an input is a valid hashtag (starts with #, contains only letters, numbers, and underscore, no space or special characters).

(1) Name your program hashtag. c.
(2) The user input ends with the user pressing the enter key (a new line character).
(3) Library function isalpha(), isdigit(), and isalnum() are not allowed in this program.
(4) Use getchar() to read in the input.

C code is given below

Explanation:

#include<stdio.h>

int main()

{

  printf("Input:");

  int ch;

  int valid=1; // it is set as 1 initially if anything is wrong it is changed to 0

  ch=getchar();

  if(ch!='#') // it checks if it starts with #

  {

      valid=0;

  }

while((ch = getchar())!= '\n') // this loop runs till line ends

  {

if((ch>='a'&&ch<='z')||(ch>='A'&&ch<='Z')||(ch>='0'&&ch<='9')||(ch=='_')) // if conditions are matched then its okay

{

  }

  else // else valid is changed to 0 from 1

  {

     valid=0;

  }

}

if(valid==0) // if anywhere there is anything wrong then it prints not valid

{

  printf("It is not valid hashtag");

  }

  else // else prints valid

  {

      printf("It is a valid hastag");

  }

}