What's wrong with this particular regular expression?

Question

I need to write a regular expression to validate the input in a form. I want to restrict the use of these characters: \ / & < > " . Everything else is allowed.

Examples of valid inputs are: My Basket, Groceries, Fruits, £$%, and +=.

Examples of invalid inputs are: A&B, A > B, 2 / 3, and A<>C.

Below is the code I'm using which is not working properly, because it returns as valid some inputs than actually are invalids.

public class Main {

    public static void main(String[] args) throws IOException {

        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

        while (true) {
            System.out.print("\nEnter text: ");
            String inputText = br.readLine();

            System.out.println("\nThe input is " + (isValidInput(inputText) ? "valid" : "invalid"));
        }
    }

    public static boolean isValidInput(String inputText) {

        Pattern p = Pattern.compile("[^/\\\\/&<>\"]");
        Matcher matcher = p.matcher(inputText);

        return matcher.find();
    }
}

@devnull yes, I can but that's not what worries me, my problem is related with the regular expression itself. — Diego Palomar, Apr 24 '14 at 13:18
With regex questions you need to add a tag for the language you are using. (Looks like Java?) — ridgerunner, Apr 24 '14 at 14:51
For future reference http://stackoverflow.com/questions/22937618/reference-what-does-this-regex-mean — Amicable, Apr 24 '14 at 15:09

Robin · Accepted Answer · 2014-04-24T14:54:32.787

Finding [^/\\\\&<>\"] will only check that at least one of the character isn't a forbidden one.

If you want to check that the whole string is made of allowed characters, you have to anchor the regex:

Pattern.compile("^[^/\\\\&<>\"]*$").matcher(inputText).find();

With ^$ matching the beginning and end of the string.

Or, as pointed out by @devnull, you can use String.matches wihch anchors the regex by default:

inputText.matches("[^/\\\\&<>\"]*")

score 1 · Answer 2 · answered Apr 24 '14 at 13:20

1

Your find will succeed if it finds any character that is not in your list, regardless of the presence of such characters in other parts of the string. Try:

"^[^/\\\\/&<>\"]*$"

answered Apr 24 '14 at 13:20

Trygve Flathen

686
7
15

score 1 · Answer 3 · answered Apr 24 '14 at 13:24

1

Uses negative lookahead to find if the string contains \ / & < > "

if (subjectString.matches("^(?!.*[\\\\/&<>\"]).*$")) {
        // VALID STRING
    } else {
        // INVALID STRING
    }

answered Apr 24 '14 at 13:24

Pedro Lobito

94,083
31
258
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What's wrong with this particular regular expression?

3 Answers3