<a href="photos.php" name="photo" id="photo1"><img src="photos/mini/1.jpg" /></a>
<a href="photos.php" name="photo" id="photo2"><img src="photos/mini/2.jpg" /></a>
<script>
var elem = document.getElementsByTagName('a'),
l = elem.length;
for (i = 0; i < l; i++) {
elem[i].id = elem[i].id.match(/\d+$/)[0]
}
for (i=0;i<l;i++) alert(elem[i].id)
</script>

var id="photo1";
alert(id.substr(id.length-1,1));

var id = 'photo23'
alert(id.substr(id.length-1,1))

var id='photo123';
alert(id.substring(5,id.length))

var id="photo1";
alert(id.substr(id.length-1,1));

var id="photo1";
alert(id.slice(-1));

var id="photo23";
alert(id.substr(5));

var id="zphoto_23";
alert(+/\d+$/.exec(id));