Java

import javax.swing.*;
import java.util.regex.Pattern;
import java.util.regex.Matcher;

public class bruce_tanner_A3Q3
{
	public static void main(String [] args)
	{
		String input;
		do
		{
			input = getString("Enter a string");
			printOutput(fixErrors(input));
		}while(checkForError(input));
	}

	//Returns true if specified string has an error ('@' in it) and false
	//if it does not.
	public static boolean checkForError(String input)
	{
		Pattern pattern = Pattern.compile("@");
		Matcher matcher = pattern.matcher(input);

		return matcher.find();
	}

	//Prompts the user for a string with the specified input
	//Returns the input string
	public static String getString(String input)
	{
		String tempString = JOptionPane.showInputDialog(input);
		return tempString;
	}

	//Prints the specified message
	public static void printOutput(String tempString)
	{
		System.out.println(tempString);
	}

	//Fixes any errors in the input string
	public static String fixErrors(String tempString)
	{
		if(checkForError(tempString))
		{
			Pattern pattern = Pattern.compile("[^@][@?+]");
			Matcher matcher = pattern.matcher(tempString);

			while(matcher.find())
			{
				tempString = matcher.replaceAll("");
				matcher = pattern.matcher(tempString);
			}

			pattern = Pattern.compile("@");				//This block gets rid of any extra @'s left over.
			matcher = pattern.matcher(tempString);
			tempString = matcher.replaceAll("");

			return tempString;
		}
		return "This string is error free";
	}
}

public static void main(String [] args)
{
	String input;													//The main method of this program only contains the input string which get's
	for(input = getString("Enter a string"); checkForError(input); input = getString("Enter a string"))	//it's value from the method 'getString'. It then
	{																//
		printOutput(fixErrors(input));
	}
	printOutput("This string is error free");
}

public static String fixErrors(String tempString)
{
	Pattern pattern = Pattern.compile("[^@][@?+]");
	Matcher matcher = pattern.matcher(tempString);
	
	while(matcher.find())
	{
		tempString = matcher.replaceAll("");
		matcher = pattern.matcher(tempString);
	}

	pattern = Pattern.compile("@");				//This block gets rid of any extra @'s left over.
	matcher = pattern.matcher(tempString);
	tempString = matcher.replaceAll("");

	return tempString;
}