Java: How can i check, if the Input matches a RegEx defined spectrum? String check works for me, but i want to check if its number or not

Question

Accidentally i postet before i was finish. Here a basic description: I'm working on a simple calculater (apprenticeship IT), and am stucked in following problem: Shurly i could get to my goal in an other way (regex-opposite match), but i'd like to understand, why this problem exists.

(I think i now reckon what's the problem. Even if i type in a letter, the scanner is trying to read the next DOUBLE. Afcourse it won't be able to compare with regex, if the error already appears before it get's to comparing :))

This works:

    OP = read.next();
    if (OP.matches(isOperand) == false){
        happend = true;
        fehlerMsg();
        askForOp();
    }

But this doesn't work:

   result = read.nextDouble(); 
    String stResult;
    stResult = String.valueOf(result);
    if (!stResult.matches(isNumber)){
        fehlerMsg();
    }

Would be super, if some one could give me a clue, why this doesn't work with the double Regex check. I just get a nothing specific saying error when i try to compile. Sorry for my bad english, i'm swiss. Here the full Code (OP is a String, result a double, stResult a string and isOperand/isNumber are RegEx:

public class TheUltimativeBestCalculatorOnThisWorldIswear {

public static boolean happend = false;
public static String isOperand = "[\\+\\-\\*\\/\\^]";
public static String isNumber = "[\\d]";
public static Scanner read = new Scanner(System.in);

public static String answer;
static String sTresult;
static boolean weiter = true;
static double zahlX;
static double zahlY;
static double result;

static boolean fehler = true;
static String OP;
/**
 * @param args the command line arguments
 */

//Start the Stuff
public static void main(String[] args) {
    first();
    LetTheShowBegin();
}


//First thing that happens
public static void first(){
    write("Zahl1?");


    result = read.nextDouble(); 
    String stResult;
    stResult = String.valueOf(result);
    if (!stResult.matches(isNumber)){
        fehlerMsg();
    }



}

//Second thing that happens
public static void second(){
    write("Zahl2?");
    zahlY = read.nextInt();
}

//Thing that happens on n' on
public static void LetTheShowBegin(){
    while (weiter){
        askForOp();
        second();
        calc();
        showTheStuff();
    }
    write("Tschüss");
}

public static void showTheStuff(){
    sTresult = String.valueOf(result);
    write(sTresult);
    write("Weiter? (j oder n)");

    answer = read.next();
    if ("j".equals(answer)){
        weiter = true;
    }

    else if ("n".equals(answer)){
        weiter = false;
    }
}

public static void askForOp(){
   if (happend == false){
    write("Welchen Operator möchten Sie benutzen?");
   }

   else if (happend == true){
    write("Nochmal: Welchen OPERATOR möchten Sie benutzen? (+, -, *, /, ^)");

   }
    OP = read.next();
    if (OP.matches(isOperand) == false){
        happend = true;
        fehlerMsg();
        askForOp();
    }


}

public static void fehlerMsg(){
    write("Versuche es noch mal, ich bin ein Waal, habe keine Wahl, mit meinem Schaal....");
}
public static void calc(){
    if (null != OP)switch (OP) {
        case "+":
            doPlus();
            break;
        case "-":
            doMinus();
            break;
        case "*":
            doTimes();
            break;
        case "/":
            doDurch();
            break;
        case "^":
            doPow();
            break;
    }
}

public static void doPlus(){
    result = result + zahlY;
}

public static void doMinus(){
    result = result - zahlY;
}

public static void doTimes(){
    result = result * zahlY;
}

public static void doDurch(){
    result = result / zahlY;
}

public static void doPow(){
    result = Math.pow(result, zahlY);
}



public static void fehler(){

}

public static void write(String x){
    System.out.println(x);
}

}

Answer 1

nextDouble() will throw an Exception if the next token is not a double :

InputMismatchException - if the next token does not match the Double regular expression, or is out of range

You may first call hasNextDouble() to ensure that there is a double value coming next.

Or, you may read the token as a String (method next()) , then apply your regex to check if it is a double.