gistfile1.java

import java.io.*;

public class Identifier
{
	public static void main(String [] args)throws IOException
	{
		String id;
		
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		System.out.println("Enter a string: ");
		id = br.readLine();
		fa(id);
		
	}
	
	public static void fa(String in)
	{
		int state = 0;
		
		for(int i = 0; i<in.length(); i++)
		{
			char foo = in.charAt(i);
			
			System.out.println("State: "+state);
			
			if(state == 0)
			{	
				if(foo == '_' || (foo >= 'A' && foo <= 'Z') || (foo >= 'a' && foo <= 'z')) 
					state = 1;
				else
					break;
			}
			else if(state == 1)
			{
				if(foo == '_' || (foo >= 'A' && foo <= 'Z') || (foo >= 'a' && foo <= 'z') || (foo >= '0' && foo <= '9'))
					state = 1;
				else{
					System.out.println("Not a valid string");
					state = -1;
					break;	
				}
			}
		}
		if(state == 0)
			System.out.println("Not an identifier");
		else if(state == 1)
			System.out.println("Identifier!");
	}
}