My code with matches() is not working. Why is it not working?

Question

I am trying to create a small program that reads a password and determines if it is a password you can use. I was using matches() to see if it has at least one letter (uppercase or lowercase) and one digit as well as be 6 characters long. I try to use matches like this:

if ( passwordLength >= 6 
   && password.matches("[A-Za-z]") 
   && password.matches("\\d")
) {
    System.out.println("Valid Password.");
} 
else {
    System.out.println("Invalid Password.");
}

Im not sure what i am doing wrong. Please help.

Avinash Raj · Accepted Answer · 2015-11-05T00:37:31.920

1

matches function should try to match the whole string using the regex we gave. So instead of using two or more matches functions for condition checking, you may use a single one with a complex regex. And also it seems like your password would contain not only digits or letters but also some other characters too.

string.matches("^(?=.*[a-zA-Z])(?=.*\\d).{6,}$");

(?=.*[a-zA-Z]) Positive lookahead which asserts that the string going to be matched must contain atleast one letter.
(?=.*\\d) asserts it must contain atleast one digit.
.{6,} ensures that the length must be atleast 6 and atmost any..

For the length to be exactly 6 then change .{6,} in the above to .{6}

DEMO

edited Nov 05 '15 at 00:37

answered Nov 05 '15 at 00:26

Avinash Raj

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This doesnt explain what was wrong with the initial approach or how this solution solves the problem – Hunter McMillen Nov 05 '15 at 00:34
I'm with @HunterMcMillen. While this is a fine solution, the "teach a person to fish" mindset suggests it would be good to explain why the original doesn't work. – blm Nov 05 '15 at 00:37

Erik Pragt · Answer 2 · 2015-11-05T00:34:12.223

0

There are a couple of things wrong with your code:

Your regex is wrong. You are only matching on 1 character, so you should change the [a-z] to [a-z]+.
You are matching on 'it should be bigger than 6, should be a character, AND it should be a number. That's different from your requirement.

edited Nov 05 '15 at 00:34

answered Nov 05 '15 at 00:26

Erik Pragt

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for the second part, he must need to use contains. Since `\\d+` checks for the string to contain digits only. – Avinash Raj Nov 05 '15 at 00:29
Maybe, but checking if a password is either only digits or only characters is a bit weird, right? Probably `^[a-zA-Z0-9]{6,}$` would make more sense in this example. – Erik Pragt Nov 05 '15 at 00:30
no, he wants the password to contain atleast one digit and atleast one letter. – Avinash Raj Nov 05 '15 at 00:31
That's basically completely wrong. The fundamental problem is that `matches()` matches the entire string, so `password.matches("[A-Za-z]")` only returns true if password is one character long and that character is an upper- or lowercase ASCII letter. On the the second bullet point, no it's not weird, because the poster want passwords to be at least 6 characters long AND contain a letter AND contain a digit. The poster's just doing the contains part wrong. – blm Nov 05 '15 at 00:31
Ah, sorry, I missed that completely, I was only looking at the code. – Erik Pragt Nov 05 '15 at 00:33

score 0 · Answer 3 · answered Nov 05 '15 at 00:32

It looks like you're misinterpreting the matches function, which matches over the entire input, while you're expecting it to return true if a substring matches. As others suggested, you'll need to use a single regex and matches() call since you're asking if the string is a single character AND a number (which would never be true).

Another important point, though it may not be important to you, is that a password should never be stored as a String object since they're immutable and can persist long enough for something else to come along and read it. More on that here.

My code with matches() is not working. Why is it not working?

3 Answers3