knoppix@Microknoppix:~$ perl -E '
> @urls = qw{
>    http://google.com/folder/1234.html
>    http://google.com/folder/abcd.html
>    };
> foreach $url ( @urls )
> {
>     $num = $url =~ m{.*/(\d+)} ? $1 : q{No match};
>     say qq{$url : $num};
> }'
http://google.com/folder/1234.html : 1234
http://google.com/folder/abcd.html : No match
knoppix@Microknoppix:~$
[download]

Update: Tightened up a bit.

knoppix@Microknoppix:~$ perl -E '
> @urls = qw{
>    http://google.com/folder/1234.html
>    http://google.com/folder/abcd.html
>    http://google.com/40lder/1234.html
>    http://google.com/40lder/abcd.html
>    };
> foreach $url ( @urls )
> {
>     $num = $url =~ m{.*/(\d+)(?!.*/)} ? $1 : q{No match};
>     say qq{$url : $num};
> }'
http://google.com/folder/1234.html : 1234
http://google.com/folder/abcd.html : No match
http://google.com/40lder/1234.html : 1234
http://google.com/40lder/abcd.html : No match
knoppix@Microknoppix:~$
[download]