String Formatting with Regex

My latest favorite trick with regular expressions is to shortcut string formatting. We’ve all written some code like this:

if (/* string is not formatted a certain way */)
{
    /* make it formatted that way */
}

Now, there’s nothing wrong with that code, but for simple examples you could do it all in one step with a regular expression!

Here are a few examples:

// remove "www." from a domain if one exists
// domain.com     -> domain.com
// www.domain.com -> domain.com
Regex.Replace(input, @"^(?:www.)?(.+)$", "$1");

// format phone number
// 1234567890       -> 123-456-7890
// (123) 456-7890   -> 123-456-7890
// (123) 456 - 7890 -> 123-456-7890
Regex.Replace(input, @"^\(?(\d{3})\)?\s*(\d{3})\s*-?\s*(\d{4})$", "$1-$2-$3");

Yay, regular expressions!