preg_replace php删除字符后

I have a URL like this : www.test.fr/dir/file.html#hello I would delete everything after the character #. I have try this /#[a-z0-9]+/

You code is almost correct, this works just fine:

$new = preg_replace('/#[a-z0-9]+/', '', 'www.test.fr/dir/file.html#hello');
print ($new);

prints:
www.test.fr/dir/file.html

You can test it here

You can explode by '#' and get the first position. Something like this:

$url = "www.test.fr/dir/file.html#hello";
$result = explode("#",$url)[0];