Merging 3 Regular Expressions to make a Slug/URL validation check

Question

I am trying to merge a few working RegEx patterns together (AND them). I don't think I am doing this properly, further, the first RegEx might be getting in the way of the next two.

Slug example (no special characters except for - and _):

(^[a-z0-9-_]+$)

Then I would like to ensure the first character is NOT - or _:

(^[^-_])

Then I would like to ensure the last character is NOT - or _:

([^-_]$)

Match (good Alias):

• my-new_page
• pagename

Not-Match (bad Alias)

• -my-new-page
• my-new-page_
• !@#$%^&*()

If this RegExp can be simplified and I am more than happy to use it. I am trying to create validation on a page URL that the user can provide, I am looking for the user to:

• Not start or and with a special character
• Start and end with a number or letter
• middle (not start and end) can include - and _

One I get that working, I can tweak if for other characters as needed.

In the end I am applying as an Annotation to my model like so:

[RegularExpression(
    @"(^[a-z0-9-_]+$)?(^[^-_])?([^-_]$)", 
    ErrorMessage = "Alias is not valid")
]

Thank you, and let me know if I should provide more information.

Answer 1

See regex in use here

^[a-z\d](?:[a-z\d_-]*[a-z\d])?$

• ^ Assert position at the start of the line
• [a-z\d] Match any lowercase ASCII letter or digit
• (?:[a-z\d_-]*[a-z\d])? Optionally match the following
  • [a-z\d_-]* Match any character in the set any number of times
  • [a-z\d] Match any lowercase ASCII letter or digit
• $ Assert position at the end of the line

See code in use here

using System; using System.Text.RegularExpressions;

public class Example
{
    public static void Main()
    {
        Regex regex = new Regex(@"^[a-z\d](?:[a-z\d_-]*[a-z\d])?$");
        string[] strings = {"my-new_page", "pagename", "-my-new-page", "my-new-page_", "!@#$%^&*()"};

        foreach(string s in strings) {
                if (regex.IsMatch(s))
                {
                    Console.WriteLine(s);
                }
        }
    }
}

Result (only positive matches):

my-new_page
pagename