Capturing optional regex segment with PHP

I need to check the end of a URL for the possible existence of /news_archive or /news_archive/5 in PHP. The below snippet does exactly what I want, but I know that I could achieve this with one preg_match rather than two. How can I improve this code to treat the /5 as an optional segment and capture it if it exists?

if (preg_match('~/[0-9A-Za-z_-]+_archive/[0-9]+$~',$_SERVER['HTTP_REFERER'], $matches) || preg_match('~/[0-9A-Za-z_-]+_archive$~', $_SERVER['HTTP_REFERER'],$matches)) {
$page_info['parent_page']['page_label'] = ltrim($matches[0], '/');
}


~/[0-9A-Za-z_-]+_archive/[0-9]+$~  Let's break it down: 1. / a literal string / 2. [0-9A-Za-z_-]+ one or more of 0-9, A-Z, a-z, _ or - 3. _archive a literal string _archive 4. / literal slash again 5. [0-9]+ one or more digits 6. $ the end of the string must follow the one or more digits

So basically you want to make #4 and #5 optional. To be more specific, you want either both 4 and 5, or neither 4 nor 5.

Consider this:

(a[b]+)?

This means that you have one a followed by one or more b, and that this grouped a/b entity is optional.

Letting a be #4 and b be digits like in #5, we're left with:

(/[0-9]+)?

Or:

~/[0-9A-Za-z_-]+_archive(/[0-9]+)?$~  This will capture the entire group though, like /5: php -r "preg_match('~/[0-9A-Za-z_-]+_archive(/([0-9]+))?$~', '/news_archive/5', $m); var_dump($m);"
array(2) {
[0] =>
string(15) "/news_archive/5"
[1] =>
string(2) "/5"
}


You can just add another group to remedy that though:

~/[0-9A-Za-z_-]+_archive(/([0-9]+))?$~  Example: php -r "preg_match('~/[0-9A-Za-z_-]+_archive(/([0-9]+))?$~', '/news_archive/44', $m); var_dump($m);"
array(3) {
[0] =>
string(16) "/news_archive/44"
[1] =>
string(3) "/44"
[2] =>
string(2) "44"
}


You could technically make the outside group a non-capturing group (like (?:/([0-9]+))?), but I don't think the added complication is worth not grabbing the / part too.

(By the way, sorry if you're familiar with regex and you found this excessive. I tend to take a very verbose approach to any regex related question :).)

• This is a fantastic response, and certainly more than I expected. In a good way. I am fairly unfamiliar with regex itself, so the thorough analysis was a pleasant and refreshing lesson! Thank you. – davo0105 Oct 5 '12 at 22:43
• @davo0105 Glad I could help! :) – Corbin Oct 6 '12 at 3:58