Restrict string input from user to alphabetic and numerical values [duplicate]

Answer 1

I don't think you can prevent the users from entering invalid values, but you have the option of validating the data you receive. I'm a fan of regular expressions. Real quick, something like this maybe (all values initialized to empty Strings):

while (!firstName.matches("^[a-zA-Z]+$")) {
    System.out.println("Please enter in a first name");
    firstName = scan.next();
}

...

while (!studentID.matches("^\\d{8}$")) {
    System.out.println("Please enter in an eight digit student ID number");
    changeID();
}

If you go this route, you might as well categorize the different cases you need to validate and create a few helper methods to deal with each.

"Regex" tends to seem overwhelming in the beginning, but learning it has great value and there's no shortage of tutorials for it.

Answer 2

It's probably easiest to do this with a regular expression. Here's some sample code:

import java.util.regex.*;

public class Test
{
    public static void main(String[] args) throws Exception
    {
        System.out.println(isNumeric("123"));
        System.out.println(isNumeric("abc"));
        System.out.println(isNumeric("abc123"));

        System.out.println(isAlpha("123"));
        System.out.println(isAlpha("abc"));
        System.out.println(isAlpha("abc123"));
    }

    private static final Pattern NUMBERS = Pattern.compile("\\d+");
    private static final Pattern LETTERS = Pattern.compile("\\p{Alpha}+");

    public static final boolean isNumeric(String text)
    {
        return NUMBERS.matcher(text).matches();
    }

    public static final boolean isAlpha(String text)
    {
        return LETTERS.matcher(text).matches();
    }
}

You should probably write methods of "getAlphaInput" and "getNumericInput" which perform the appropriate loop of prompt/fetch/check until the input is correct. Or possibly just getInput(Pattern) to avoid writing similar code for different patterns.

You should also work out requirements around what counts as a "letter" - the above only does a-z and A-Z... if you need to cope with accents etc as well, you should look more closely at the Pattern docs and adapt appropriately.

Note that you can use a regex to validate things like the length of the string as well. They're very flexible.

Answer 3

try regexp: \d+ -- numerical, [A-Za-z]+ -- alphabetical

Answer 4

java.util.Scanner can already check if the next token is of a given pattern/type with the hasNextXXX methods.

Here's an example of using boolean hasNext(String pattern) to validate that the next token consists of only letters, using the regular expression [A-Za-z]+:

    Scanner sc = new Scanner(System.in);
    System.out.println("Please enter letters:");
    while (!sc.hasNext("[A-Za-z]+")) {
        System.out.println("Nope, that's not it!");
        sc.next();
    }
    String word = sc.next();
    System.out.println("Thank you! Got " + word);

Here's an example session:

Please enter letters:
&#@#$
Nope, that's not it!
123
Nope, that's not it!
james bond
Thank you! Got james

To validate that the next token is a number that you can convert to int, use hasNextInt() and then nextInt().

Related questions

• Validating input using java.util.Scanner - has many examples!

Answer 5

Im not sure this is the best way to do, but you could use Character.isDigit() and Character.IsLiteral() mabybe like this:

for( char c : myString.toCharArray() ) {
    if( !Character.isLiteral(c) ) {
        // 
    }
}

Answer 6

That is the code

    public class InputLetters {
    String  InputWords;
    Scanner reader;
    boolean [] TF;
    boolean FT;     

    public InputLetters() {

        FT=false;

        while(!FT){     
            System.out.println("Enter that you want to: ");
            reader = new Scanner(System.in);
            InputWords = reader.nextLine();
            Control(InputWords);            
        }

    }


public void Control(String s){
String [] b = s.split(" ");
TF = new boolean[b.length];
    for(int i =0;i<b.length;i++){
        if(b[i].matches("^[a-zA-Z]+$")){
            TF[i]=true;
        }else
        {
            TF[i]=false;
        }               
    }
    for(int j=0;j<TF.length;j++){
        if(!TF[j]){
            FT=false;
            System.out.println("Enter only English Characters!");
            break;
        }else{
            FT=true;
        }

    }
}