Method Matches() to validate if string contains only numbers

Question

I’ve read some articles on the method matches() and really didn’t understand.

In that code would return false because I am using a variable and the method read the variable name and not the value referenced by it.

String seq ="7865";
boolean Numeros = "seq".matches("\\d"); 
System.out.println("Possui números? "+Numeros);

In this other would return true because the number itself "calls" the method and only has a number. When it has more than 1 number returns false.

boolean Numeros = "1".matches("\\d"); 
System.out.println("Possui números? "+Numeros);

Could someone explain me the function of this method? I am looking for some method to use in CPF validation. To receive only numbers. It’s hard.

Answer 1

The first code doesn’t work because "seq" (in quotes) is a String which contains the text "seq", and has nothing to do with the variable seq.

That is to say, "seq".matches("\\d") is checking whether the String (the text) "seq" contains a number. But like this String only has letters (s, e and q), the return is false.

If you want to check the contents of the variable seq, remove the quotes and use the variable itself directly:

String seq ="7865";
boolean numeros = seq.matches("\\d"); // <-- repare que seq está sem as aspas
System.out.println("Possui números? " + numeros);

But the above code will still return false, because the method matches checks whether the String integer matches regex. How regex is \\d, it only checks if the String contains one digit (exactly one, so the second code with "1".matches("\\d") works).

Anyway, if you want to validate that the String whole only has numbers from start to finish, use:

boolean numeros = seq.matches("^\\d+$"); 

Now we have the markers ^ and $, which are respectively the beginning and end of the string, and the quantifier +, which indicates "one or more occurrences". That is, regex checks whether the String has only one or more digits from start to finish.

You could also place specific quantities if you like. Examples:

• \\d{11}: exactly 11 digits
• \\d{4,11}: not less than 4, not more than 11 digits
• \\d{4,}: at least 4 digits, with no upper limit

Adjust the values for whatever you need.

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But as you said you want to validate CPF, regex is not the best solution. Regular expressions always work with text, and even digits are treated as mere characters, regardless of their numerical value. regex will only check if the string has only digits, but CPF validation also includes calculations to check the check digits, and that is much easier to do without regex. Though it has its usefulness, regex is not always the best solution.