Regex pattern for checking if a string starts with a certain substring?

What's the regular expression to check if a string starts with "mailto" or "ftp" or "joe" or...

Now I am using C# and code like this in a big if with many ors:

String.StartsWith("mailto:")
String.StartsWith("ftp")

It looks like a regex would be better for this. Or is there a C# way I am missing here?

You could use:

^(mailto|ftp|joe)

But to be honest, StartsWith is perfectly fine to here. You could rewrite it as follows:

string[] prefixes = { "http", "mailto", "joe" };
string s = "joe:bloggs";
bool result = prefixes.Any(prefix => s.StartsWith(prefix));

You could also look at the System.Uri class if you are parsing URIs.

The following will match on any string that starts with mailto, ftp or http:

 RegEx reg = new RegEx("^(mailto|ftp|http)");

To break it down:

• ^ matches start of line
• (mailto|ftp|http) matches any of the items separated by a |

I would find StartsWith to be more readable in this case.

The StartsWith method will be faster, as there is no overhead of interpreting a regular expression, but here is how you do it:

if (Regex.IsMatch(theString, "^(mailto|ftp|joe):")) ...

The ^ mathes the start of the string. You can put any protocols between the parentheses separated by | characters.

edit:

Another approach that is much faster, is to get the start of the string and use in a switch. The switch sets up a hash table with the strings, so it's faster than comparing all the strings:

int index = theString.IndexOf(':');
if (index != -1) {
  switch (theString.Substring(0, index)) {
    case "mailto":
    case "ftp":
    case "joe":
      // do something
      break;
  }
}