Here’s a better way:

http://api.instagram.com/oembed?url=http://instagram.com/p/Y7GF-5vftL/

Render as json object and you can easily extract media id from it —

For instance, in PHP

$api = file_get_contents("http://api.instagram.com/oembed?url=http://instagram.com/p/Y7‌​GF-5vftL/");      
$apiObj = json_decode($api,true);      
$media_id = $apiObj['media_id'];

For instance, in JS

$.ajax({     
    type: 'GET',     
    url: 'http://api.instagram.com/oembed?callback=&url=http://instagram.com/p/Y7GF-5vftL‌​/',     
    cache: false,     
    dataType: 'jsonp',     
    success: function(data) {           
        try{              
            var media_id = data[0].media_id;          
        }catch(err){}   
    } 
});