Validation using Regex

Question

I have a validation like, String should contain at least one alphabet, or number. It may contain _ or - as optional character in between. The string should start and end with an alphabet or a number. I tried using many Regex, but I can't achieve this. Here is my RegEx code :

public static boolean isValidURL(String inputString) {
    try {
        boolean isValid = true;
        Pattern letter = Pattern.compile("[a-zA-z0-9]");
        String restPattern = "^[a-zA-Z0-9]+[a-zA-Z0-9_\\-\\s]?[a-zA-Z0-9]+$";
        Matcher hasLetter = letter.matcher(inputString);
        if (hasLetter.find()) {
            if (inputString.matches(restPattern)) {
                isValid = true;
            } else {
                isValid = false;
            }
        } else {
            isValid = false;
        }
        return isValid;
    } catch (Exception e) {
        throw e;
    }
}

And My Unit Test :

    assertEquals(true, Validator.isValidURL("res"));
    assertEquals(true, Validator.isValidURL("res_rer"));
    assertEquals(true, Validator.isValidURL("res-rer"));
    assertEquals(true, Validator.isValidURL("res232A"));
    assertEquals(true, Validator.isValidURL("232DFA"));
    assertEquals(true, Validator.isValidURL("23_323"));
    assertEquals(true, Validator.isValidURL("23"));
    assertEquals(true, Validator.isValidURL("A2s"));
    assertEquals(false, Validator.isValidURL("_"));
    assertEquals(false, Validator.isValidURL("-"));
    assertEquals(false, Validator.isValidURL("@"));
    assertEquals(false, Validator.isValidURL("@GR$"));
    assertEquals(false, Validator.isValidURL("_GR_"));
    assertEquals(false, Validator.isValidURL("GR_"));
    assertEquals(false, Validator.isValidURL("GR_"));
    assertEquals(true, Validator.isValidURL("s"));
    assertEquals(true, Validator.isValidURL("4"));

Last two assert getting failed. Please help me to solve this issue.

The regex ends with a + quantifier so an extra character is needed at the end and since the last two strings are of length one it fails. — xtreak
– xtreak, Commented Sep 19, 2014 at 4:34
If I put optional quantifier ? ^[a-zA-Z0-9]?[a-zA-Z0-9_\\-\\s]?[a-zA-Z0-9]+$ The second assert itself getting failed. — Pasupathi Rajamanickam
– Pasupathi Rajamanickam, Commented Sep 19, 2014 at 4:35

Avinash Raj · Accepted Answer · 2014-09-19 04:49:05Z

4

The below regex would allow one or more alphanumeric characters and also allows - or _ in-between the alphanumeric chars but not at the first or at the last.

^[a-zA-Z0-9]+(?:[-_]+[a-zA-Z0-9]+)*$

DEMO

If you don't want consecutive _ or - then remove the + following the character class which was present inside the non-capturing group.

^[a-zA-Z0-9]+(?:[-_][a-zA-Z0-9]+)*$

edited Sep 19, 2014 at 4:49

answered Sep 19, 2014 at 4:35

Avinash Raj

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4 Comments

nhahtdh Over a year ago

This is going to run into backtracking hell. Remove the optional quantifier ?

Pasupathi Rajamanickam Over a year ago

assertEquals(true, UtilityFunction.isValidURL("23___323")); This assert getting failed.

Avinash Raj Over a year ago

It may contains _ and - as optional char in between

Avinash Raj Over a year ago

just add a plus ^[a-zA-Z0-9]+(?:[-_]+[a-zA-Z0-9]+)*$ see regex101.com/r/gG9vW6/8

nhahtdh · Accepted Answer · 2014-09-19 04:43:21Z

2

This should restrict the input to start and end with alphanumeric, and allow multiple consecutive - and _ within the input.

^[a-zA-Z0-9]++(?:[-_]++[a-zA-Z0-9]++)*+$

I turn all the quantifiers possessive (++ and *+ instead of + and *), to disallow backtracking.

answered Sep 19, 2014 at 4:43

nhahtdh

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1 Comment

nhahtdh Over a year ago

@AvinashRaj: I develop the solution independently, but slower in answering the question, since I need to get all the assumptions by the OP straight.

Lijo · Accepted Answer · 2014-09-19 04:44:45Z

0

^([a-zA-Z0-9]+[a-zA-Z0-9_\\-\\s]?[a-zA-Z0-9]+)|([a-zA-Z0-9])$

Just add one more or condition which accepting either alphabet or number

answered Sep 19, 2014 at 4:44

Lijo

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1 Comment

nhahtdh Over a year ago

Correctness aside, this will do useless backtracking, since [a-zA-Z0-9_\\-\\s]? is optional, and you have [a-zA-Z0-9]+[a-zA-Z0-9]+ in a row.

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