Why doesn't any regex constructor understand this?

Question

PHP

~^/forum(/[a-z0-9_/\-]+/)([0-9]+)\.xhtml$~
Line: /forum/web-development/php/12345.xhtml

Answer 1

because constructors usually use the format /regex/, and your regular expression crashes because of unescaped slashes.
^\/forum(\/[\d\w_\/\-]+\/)([\d]+)\.xhtml

Answer 2

by hand and with the help of these pseudo-classes:

:last-child
:nth-child(n)

google