Regex that does not match any html tag

Question

I am really bad with regexps, what I want is a regexp that does not match any html tags (for user input validation).

What I want is negative of this:

<[^>]+>

What I currently have is

public class MessageViewModel
{
    [Required]
    [RegularExpression(@"<[^>]+>", ErrorMessage = "No html tags allowed")]
    public string UserName { get; set; }
}

but it does opposite of what I want - allows usernames only with html tags

Answer 1

Regular expressions cannot do "negative" matches.

But they can do "positive" matches and you can then throw out of the string everything that they have found.

---

Edit - after the question was updated, things became a little clearer. Try this:

public class MessageViewModel
{
    [Required]
    [RegularExpression(@"^(?!.*<[^>]+>).*", ErrorMessage = "No html tags allowed")]
    public string UserName { get; set; }
}

Explanation:

^            # start of string
(?!          # negative look-ahead (a position not followed by...)
  .*         #   anything
  <[^>]+>    #   something that looks like an HTML tag
)            # end look-ahead
.*           # match the remainder of the string