Question

I need to use C# to write a Regular Expression for tags, here is the requirement:

1. Tags are optional;
2. Maximum 9 tags;
3. no duplicated tags;
4. Maximum 30 characters for each tag;
5. Each tag should only contain reasonable characters (how to define?, I’m now using [\w-]);
6. Tags are separated by , (with or without one whitespace after the comma are optional);
7. The ending , (with or without one whitespace) is also legal.

I have already had this:

^(|[\w-]{1,30}(,\s?[\w-]{1,30}){0,8}(,\s?)?)$

I find it’s difficult to meet all the rules. especially rule 3.

EDIT:

1. Updated rule 5, rule 6;
2. Applied rule 7.

Answer 1

Do it without Regex:

public static bool TryGetTags(string tagsInput, out string[] tags)
{
    Regex regex = new Regex(@"^[\w_-]+$");

    tags = tagsInput.Split(',')  // Rule 6
                    .Select(tag => tag.Trim())
                    .ToArray();

    if (tags.Last() == "")
        tags = tags.Take(tags.Length - 1).ToArray();  // Rule 7

    if (tags.Any(tag => tag == ""))  // (no empty tag allowed except last one)
        return false;

    if (tags.Length > 9)
        return false;  // Rule 2

    if (tags.Any(tag => tag.Length > 30))
        return false;  // Rule 4

    if (tags.Distinct().Count() != tags.Length)
        return false;  // Rule 3

    if (tags.Any(tag => !regex.IsMatch(tag)))
        return false;  // Rule 5

    return true;
}