How to Write a Regular Expression that allows only letters, numbers and special characters only i the middle never in the end

Question

Need some regular expressions help.

So far I have my code working to allow a limited number of special characters. However, I don't know how to get it to only allow them in the middle and never the end.

Can someone help me figure that part out?

Here is the code I am using in c#:

            Regex uRLToVal= new Regex("^[A-Za-z0-9-_.+!*]*$");

            if (!uRLToVal.IsMatch(this.mainURL))
            {
                results.AddPropertyError("Your Entry can only contain letters, numbers, underscores, periods, plus, exclamation marks and hypens. Special characters should always be inside numbers or letters.  Example: v!v is OK BUT NOT !vv or vv!");
            }

Answer 1

new Regex(@"^[A-Za-z0-9]+([-_.+!*]+[A-Za-z0-9]+)*$");

which means :

match must begin with at least one numeric or alpha char: [AZ-z0-9]+

That sequence may be followed by zero or many special chars -_.+!* followed by at least one numeric or alpha char. This second combination can happen zero or many times (example : asdf-e*r!asdf , or a are valid)