How to use ereg_replace to convert image link to display image

Question

How can i use ereg to replace URL of images with tag.
My images URL have two starts embraced like this **http://myimageslink/photo.jpg**
i want to convert to <img src="http://myimageslink/photo.jpg"></img>

I use ** embrace because when don't have ** https://www.google.com/?gws_rd=ssl i will convert to <a href="https://www.google.com/?gws_rd=ssl">https://www.google.com/?gws_rd=ssl</a> like this.

The code below doesn't work i got like this <img src="**http://myimageslink/photo.jpg**"></img>

   $text = ereg_replace("**[[:alpha:]]+://[^<>[:space:]]+[[:alnum:]/]**",
                             "<img src=\"\\0\"></img>", $str);

but I want the results like this.

<img src="http://myimageslink/photo.jpg"></img>

And this the real example.

$text  = 'This is first photo **http://myimageslink/photo.jpg**<br />
        This is second photo **http://myimageslink/photo.jpg**';
$text  = ereg_replace("**[[:alpha:]]+://[^<>[:space:]]+[[:alnum:]/]**",
                             "<img src=\"\\0\"></img>", $text);

So, $text should be

$text = 'This is first photo <img src="http://myimageslink/photo.jpg"><br />
        This is second photo </img><img src="http://myimageslink/photo.jpg"></img>';