Regex to restrict UPPERCASE only

Question

I am stil struggling with understanding how Regex works exactly.

I am getting usernames and they are in this format:

firstname.lastname

Both names may contain special international characters and they may contain an ' or - but I just need to detect if they contain any uppercase letters so I can throw an exception.

I am using this expression

[^A-Z].[^A-Z]

It seems to me that this should work, I just don't understand why it doesn't.

I hope somebody could explain.

Thanks!

Solution 1

[^A-Z] Simply means any character that isn't a capital A through capital Z.

. Means any character you should be using \. As this means the literal character .

A character group is [] and the inverse is [^] you then put the characters you want to match.

However, your regex looks like it will match only a single character that isn't a capital letter then any character then another single character that isn't a capital letter

You want to use the following:

[^A-Z]+\.[^A-Z]+

The + in regex means match the before stated 1 to infinite times.

If you are only going to have this text and no other text you should include the start of line and end of line tag so that it doesn't match long strings that include something formatted like you mentioned.

However, your regex does also match spaces and tabs.

So I would use the following:

^[^A-Z\s]+\.[^A-Z\s]+$

Regex Demo working with only lowercase

Regex Demo failing because username has uppercase letter

Solution 2

Instead of using regex you could use this method to check for upper case characters.

public static bool checkStringForUpperCase(string s) 
{
    for (int i = 0; i < s.Length; i++)
    {
        if (char.IsUpper(s[i]))
            return false;
    }
    return true;
}

Solution 3

If you want to check that there is no uppercase, you don't need dot int middle, you can use just [^A-Z] You should use start and end regex symbols and sign that this can be more then one symbol. If i remember correctly it should be something like ^[^A-Z]*$

Solution 4

Obviously the only correct answer is to use \p{Lu} to match an uppercase Unicode letter. There are other uppercase letters in national alphabets other than [A-Z].

View more solutions

Comments

[SwDevMan81]

Almost, just add the escape you mentioned \.

[abc123]

lol, believe it or not i had a \. but the <code> tag made it .

[BradW]

Not sure why I would get voted down when I was just offering an alternative solution to which someone may find useful if they're in the same situation.

[Ehsan]

@downvoter can you please comment why you downvoted? what is wrong with the answer?

[laitha0]

I had to escape it by using \\. and \\s, is this a C# thing? it complains if I use a single \ but it works for a small test case that I have

[Anirudha]

i have not downvoted but would your code work! char.IsUpper should be x=>char.IsUpper(x)..Aksi @ is unnecessary..

[abc123]

@user2247823 yes or you can do this @"^[^A-Z\s]+\.[^A-Z\s]+$" which allows the escape character without escaping \ is an escape character in c#. that's why you need \\ that's fine to keep like that

[Hannele]

@user2247823 If you're using \ in a string, then it's probably complaining about not recognizing the escape sequence. Using \\ tells the C# compiler that you want a literal \ in your string.

[abc123]

The only issue I see with this is it doesn't do any validation other than UPPERCASE. So it doesn't even check for the period in the middle and will allow for white space as a space isn't capital.

[abc123]

The only issue I see with this is it doesn't do any validation other than UPPERCASE. So it doesn't even check for the period in the middle and will allow for white space as a space isn't capital.

[Ehsan]

@abc123 agree. that is why i wrote "If you only want to check whether it contains uppercase "

[BradW]

That is true, although you may also use the string.contains to see if those are included within the string.

[abc123]

that would only verify that one or more periods are in the string in any place.