How to find regexp in a string and attach it to a variable

Let's say we have a string开发者_如何学运维 in JavaScript "This is a nice website - http://stackoverflow.com". I want to extract the URL along with the three preceding characters (space dash space) using RegExp and attach the extracted string to a variable.

var string = "This is a nice website - http://stackoverflow.com";

var reg = ""; //no idea how to write this regexp for extracting url and three preceding chars

// and after some magic I would get

var extracedString = " - http://www.stackoverflow.com";

Anyone? Thanks.

var extractedString = string.replace(/^.*(...http:.+)$/, "$1");

if (extractedString == string) {
  alert("No match");
}

The dot . matches every character, so three dots match three arbitrary characters. The ^ and $ match start and end of the string.

Note, that this won't work for

• more than one URL

• HTTPS, mailto, FTP, SSH, ... (although you can simply expand it, like this: (https?|ftp|ssh))