Matching @ as the first character

Question

I'm building two regex helpers.

The first replaces any links with an anchor tag. Here's how it looks:

String.prototype.parseURL = function() {
  return this.replace(/[A-Za-z]+:\/\/[A-Za-z0-9-_]+\.[A-Za-z0-9-_:%&~\?\/.=@]+/g, function(url) {
    return url.link(url);
  });
};

The second replaces any Twitter handles (starting with @) with an anchor tag that points to the appropriate Twitter profile. Here's how it currently looks:

String.prototype.parseUsername = function() {
  return this.replace(/\s[@]+[A-Za-z0-9-_]+/g, function(u) {
    var username = u.replace("@","")
    return u.link("http://twitter.com/"+username);
  });
};

Both of these prototype methods are then chained on a string, which replaces the appropriately matched inputs. A previous edge case that I fixed involved the @ symbol within a hyperlink.

There's one edge case that I'm not meeting, which is when a Twitter handle is at the beginning of a string (with no characters in front of it, no whitespace, etc).

How can I have parseUsername match any instances of @ that do not have any characters such as slashes/tags/hyphens/etc. in front of it but are the first instance in the first word of a string?

Here's an image of what's going on:

Answer 1

/(?:^|\s)[@]+[A-Za-z0-9-_]+/

   ^^

or it with start of string.