New to Java

I am working on a java program that converts roman numbers. The user inputs a character string and I am needing to check to make sure that it only contains MDCLXVI and if it doesn't I will have them input the string again. I was thinking of doing a while loop but I am not sure if that is the smart way to do it or the syntax to do the check.

Here was one thought on checking with a while loop, but I have never used it before and may be building it wrong.

while (!Pattern.matches("MDCLXVI", firstRoman))
        {
        System.out.print("Not a valid Roman Number please try again");
        firstRoman = firstRoman.toUpperCase();
        }

My Code is as follows:

// Ask for user to input firstRoman Number
        System.out.print("Enter your first Roman Number:");
        // add the Roman Number to the String variable
        firstRoman = keyboard.nextLine();
        firstRoman = firstRoman.toUpperCase();

Once they enter it, I want to check to ensure that it only has the characters MDCLXVI and if it contains any other characters then I will provide them with a new Scanner Keyboard object to try again and if it fails again, then they will keep getting the option to try again until they put in a string that only contains the above characters. I just don't know where to go from here to get the while loop worked out. New to Java and I got my program to work fine when I put known good Roman Numbers in, but I want to catch incorrect entries.

Any assistance would be appreciated.

Thanks
Wally

I have put the code for my main program and my class below just in case. I have already met the teacher requirements so the project is done, I am just trying to improve on it.

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
package myname.project.pkg6;

import java.util.Scanner;
import java.util.regex.Pattern;

/**
 *
 * @author wsteadma
 */
public class mynameProject6 {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        
        // Used for testing
        String firstRoman;
        String secondRoman;
        
        
        Scanner keyboard = new Scanner(System.in);

        
        // Ask for user to input firstRoman Number
        System.out.print("Enter your first Roman Number:");
        // add the Roman Number to the String variable
        firstRoman = keyboard.nextLine();
        firstRoman = firstRoman.toUpperCase();
        
              
        // Ask for user to input secondRoman Number
        System.out.print("Enter your second Roman Number:");
        // add the Roman Number to the String variable
        secondRoman = keyboard.nextLine(); // Add the name input into the name variable
        secondRoman = secondRoman.toUpperCase();
        
        
        //Creating first instance of Roman Numbers class
        Roman firstNumber = new Roman(firstRoman);
        Roman secondNumber = new Roman(secondRoman);
        
        System.out.println("Printout of the created roman numbers");
        //Prints out the Roman Number
        firstNumber.printRoman();
        secondNumber.printRoman();
        
        
        // Convert Roman to Decimal
        firstNumber.setDecimal(firstRoman);
        secondNumber.setDecimal(secondRoman);
        
        System.out.println("Viewing the Decimal values for the two defined instances");
        //Prints decimal Numbers
        firstNumber.printDecimal();  
        secondNumber.printDecimal();
        
        System.out.println("Adding the Roman Numbers and returning new Roman value");
        Roman romanAdd;
        romanAdd = firstNumber.addRoman(secondNumber);
        romanAdd.printRoman();

        System.out.println("Subtracting the Roman Numbers and return new Roman value");
        Roman romanSub;
        romanSub = firstNumber.subRoman(secondNumber);
        romanSub.printRoman();
    }
}

CLASS:

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
package myname.project.pkg6;

/**
 *
 * @author wsteadma
 */
public class Roman {

    private String romNumber;
    private int decNumber;

    public Roman(String rNumber) {
        romNumber = rNumber;
    }

    public String getRoman() {
        return romNumber;
    }
    
    public int getDecimal()
    {
    return decNumber;
    }

    public void printRoman() {
        System.out.println("The Roman Number is " + romNumber);
    }

    public void printDecimal() {
        System.out.println("The Decimal Number is " + decNumber);
    }

    // Converts a roman number string into a decimal value
    public int setDecimal(String romNum) {
        int charValue = 0;
        char[] characters = new char[romNum.length()];

        //System.out.println("romNum length is " + romNum.length());
        for (int counter = 0; counter < romNum.length(); counter++) {
            characters[counter] = romNum.charAt(counter);
            //System.out.println(characters[counter]);
        }

        for (int sCounter = 0; sCounter < romNum.length(); sCounter++) {
            if (characters[sCounter] == 'M'  || characters[sCounter] == 'm') {
                charValue += 1000;
            } else if (characters[sCounter] == 'D' || characters[sCounter] == 'd') {
                charValue += 500;
            } else if (characters[sCounter] == 'C' || characters[sCounter] == 'c') {
                charValue += 100;
            } else if (characters[sCounter] == 'L' || characters[sCounter] == 'l') {
                charValue += 50;
            } else if (characters[sCounter] == 'X' || characters[sCounter] == 'x') {
                charValue += 10;
            } else if (characters[sCounter] == 'V' || characters[sCounter] == 'v') {
                charValue += 5;
            } else if (characters[sCounter] == 'I' || characters[sCounter] == 'i') {
                charValue += 1;
            }
        
        }
        decNumber = charValue;
        return charValue;
    }

    // Converts a decomal number to a roman number
    public String setDec2Roman(int decNum) {
        String newRoman = "";

        while (decNum >= 1000) {
            newRoman += "M";
            decNum -= 1000;
            //System.out.println(decNum);
            //System.out.println(newRoman);
        }

        while (decNum >= 500) {
            newRoman += "D";
            decNum -= 500;
            //System.out.println(decNum);
            //System.out.println(newRoman);
        }

        while (decNum >= 100) {
            newRoman += "C";
            decNum -= 100;
            //System.out.println(decNum);
            //System.out.println(newRoman);
        }

        while (decNum >= 50) {
            newRoman += "L";
            decNum -= 50;
            //System.out.println(decNum);
            //System.out.println(newRoman);
        }

        while (decNum >= 10) {
            newRoman += "X";
            decNum -= 10;
            //System.out.println(decNum);
            //System.out.println(newRoman);
        }

        while (decNum >= 5) {
            newRoman += "V";
            decNum -= 5;
            //System.out.println(decNum);
            //System.out.println(newRoman);
        }

        while (decNum > 0) {
            newRoman += "I";
            decNum -= 1;
            //System.out.println(decNum);
            //System.out.println(newRoman);
        }
        return newRoman;
    }

    public Roman addRoman(Roman newRom) {
        int totRom = newRom.setDecimal(newRom.romNumber) + decNumber;
        romNumber = newRom.setDec2Roman(totRom);
        return new Roman(romNumber);
    }
    
    public Roman subRoman(Roman subRom) {
        int totRom = decNumber - subRom.setDecimal(subRom.romNumber);
        romNumber = subRom.setDec2Roman(totRom);
        return new Roman(romNumber);
    }
    
}