CODEWITHSUNDEEP
c#regex
AaA
AaA

Reputation: 3694

How Can I forbid repeated characters using regular Expression

How Can I forbid repeated characters using regular Expression?

This regular Expression should not allow 

1--234567890
1--2345--1212

However following is valid

1-2-3-4-5-6-7-8-9-0
1234567890

The only concern here is minus cannot be entered after each other, so -- should not match

There is no restriction in how many dash are in string. I'm using C#

Thanks in Advance

Upvotes: 3

Views: 5458

Answers (4)

bluepnume
bluepnume

Reputation: 17128

Use a negative lookahead: ^((.)(?!\2))+$ will match a string with no repeated characters.

Alternatively -- potentially faster: do a search for (.)\1 which will match a pair of repeated characters.

Upvotes: 2

Oded
Oded

Reputation: 499002

This would match any pair of identical characters:

"(.)\1"

Alternatively, for word characters:

"(\w)\1"

So, if the regex matches, you want to fail validation.

Update:

Now that you clarified that only -- is what you need to match on, here is another option:

"--"

Of course, you can simply use Contains("--") on the string in such a case.

Upvotes: 4

user7116
user7116

Reputation: 64068

If a dash is the only character you care about:

// this regular expression is "inexact", three dashes always has two dashes, etc
var bad = new Regex("--");
if (bad.IsMatch(input))
{
    throw new ArgumentException("Not a valid format", "input");
}

Or the far simpler:

// regular expression? no need
if (input.Contains("--"))
{
    throw new ArgumentException("Not a valid format", "input");
}

Upvotes: 1

escargot agile
escargot agile

Reputation: 22379

This task is not suitable for regular expressions. What you probably want to do is write a loop that would go over the string and search for duplicates. The most efficient way of doing it is using some hash-based data structure like a dictionary.

Upvotes: 0

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