Help with regular expression

Hi All,

I'm having trouble with regular expressions, can anyone please help.
I'm trying to convert a string into a regular expressions, but my expression is to broad and I need to narrow it down.

if I get a string "*.domain.co.za/*" I'm converting it to a regular expresson as such:

var expressionList = new List<Regex>();

var myString = "*.domain.co.za/*";

if (myString.IndexOf('*') >= 0)
{
    expressionList.Add(new Regex(myString.Replace("*.", ".*\\.").Replace("/*", "/.*")));
}

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the thing is this is far to broad how can I change the expression to validate to the following:

1.

Must start with http or https followed by ://

2.

Any number of char or numbers (no periods ".")

3.

myString (.domain.co.za/)

4.

anything else
With these rules applied I think only the following will pass:
http://www.domain.co.za
http://www.domain.co.za/something/somethin
http://sub1.domain.co.za
http://sub2.domain.co.za/something/somethin

Is it possible to also allow http://domain.co.za to pass.

The following should not pass:
ftp://www.domain.co.za
http://www.asdg.domain.co.za
http://www.otherDomain.co.za

If it is possible please explain the expression.