Display an xkcd

Perl + curl + feh, 86 84 75 bytes

`curl xkcd.com/$_/`=~/<img src="(.*)" title="(.*?)"/;$_=$2;`feh "http:$1"`

Requires the -p switch. I have accounted for this in the byte count.

PowerShell v3+ 110 99 107 103 Bytes

iwr($x=((iwr "xkcd.com/$args").images|?{$_.title})).src.Trim("/") -outf x.jpg;if($x){ii x.jpg;$x.title}

Thanks to Timmy for helping save some bytes by using inline assignments.

If no arguments are passed then $args is null and it will just get the current comic. Download the picture, by matching the one with alt text, into a file in the current running directory of the script. Then display it with the default viewer of jpg's. The alt text is then displayed to console. iwr is an alias for Invoke-WebRequest

If the number passed (or any invalid input for that matter) does not match the process fails with at least a 404 error.

iwr(                                  # Request the comic image from XKCD
  $x=((iwr "xkcd.com/$args").images|  # Primary search to locate either the current image
                                      # or one matching an argument passed
     ?{$_.title}))                    # Find the image with alt text
        .src.Trim("/")                # Using the images associated link and strip the leading slashes
  -outf x.jpg                         # Output the image to the directory local to where the script was run
if($x){                               # Test if the image acquisition was successful
    ii x.jpg                          # Open the picture in with the default jpg viewer
    $x.title                          # Display alt text to console
}                                     # I'm a closing bracket.

AutoIt, 440 bytes

Yes, it's long, but it's stable.

#include<IE.au3>
#include<GDIPlus.au3>
Func _($0='')
_GDIPlus_Startup()
$1=_IECreate('xkcd.com/'&$0)
For $3 In $1.document.images
ExitLoop $3.title<>''
Next
$4=_GDIPlus_BitmapCreateFromMemory(InetRead($3.src),1)
$6=_GDIPlus_ImageGetDimension(_GDIPlus_BitmapCreateFromHBITMAP($4))
GUICreate(ToolTip($3.title),$6[0],$6[1])
GUICtrlSendMsg(GUICtrlCreatePic('',0,0,$6[0],$6[1]),370,0,$4)
_IEQuit($1)
GUISetState()
Do
Until GUIGetMsg()=-3
EndFunc

First of all, this doesn't use RegEx to scrape the site (because I have no time to test this on all comics), but rather uses the Internet Explorer API to iterate through the DOM's img tags until it finds one with a title text.

The binary stream is read from the image URL and rendered into a bitmap using GDIPlus. This is then displayed in a nice, auto-sized GUI with an actual tooltip to make it behave almost exactly like the website.

Here's a test case (_(859)):

)