How to implement Regex in Java

Question

I have a doubt with a problem that I have to resolve in Java. The thing is: my program receives two arguments by console, the first one is the file's path where I have to load this file to the java program, It's a .ini file. The second argument is a code, with this pattern: XXX-XXXX or XXX-XXXX-XXXX where 'X' are numbers from 0 to 9. Here in the second argument my problem comes in: I have to use the packet java.util.regex to implement this pattern, I mean, I have to validate that the user input the arguments properly. How can I do this? The part of my code that implements this is the following, thanks in advance:

Pattern patron = Pattern.compile("^[0-9]{3,}"); //aqui va la regex que queremos poner
    Matcher match = patron1.matcher(args[1]);


    if(args.length < 2){ // el programa espera recibir dos argumentos

        System.out.println("ERROR, número de argumentos inválido!");
        System.exit(1); //termina el programa
    } else if(args[0].endsWith("config.ini") == false){ //COMPLETAR la segunda condicion
        System.out.println("ERROR, los argumentos no toman valores válidos");
        System.exit(1);
    }

Titus · Accepted Answer · 2018-02-08 18:10:08Z

1

You can use something like this:

if(args.length >= 2 && args[1] != null && args[1].matches("^\\d{3,}(\\-\\d{3,}){1,2}$"))

^ stands for start of the string

\\d{3,} a number repeating 3 or more times

(\\-\\d{3,}) a group composed of a dash and the construct explained above

{1,2} means that group repeating between one and two times

$ the end of the string

edited Feb 8, 2018 at 18:10

answered Feb 8, 2018 at 18:00

Titus

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4 Comments

victor26567 Over a year ago

Please, would you explain me a little bit the regex that you used? Thanks a lot

Titus Over a year ago

@user71209 I've edited my answer to try to explain the regex. You can find a better explanation HERE

victor26567 Over a year ago

much better! The only doubt is why do you put a double backslash, it is because we are in Java? I mean, in the page regex101 its only neccessary one backslash, for instance: \d{3} --> a three digit number

Titus Over a year ago

@user71209 Yes, other languages have regex literals and you can use only one slash but in Java, because we're using String literals, we have to escape the slash.

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