Sometimes you want to match something optionally, meaning it can be there or not and it will still match. You do this with the '?' (question-mark) character. Simply put it after the regex symbol or set you want to be optional and you'll see it match. Here's a few URLs to try this on:

/blog/article/1 /blog/article/ /blog/article/2

Just a few URLs but imagine you want to match any of those as being the same thing. To do that you just put a '?' at the end and it will optionally match the [0-9] character set. I'm going to write one regex but it's in verbose mode so I can comment on each character and you can see it:

^/blog/article/[0-9]?$

^     # from the start
/blog/article/
[0-9] # any of 0-9
?     # optional though
$     # to the end

First I have the regex like normal, then I wrote it out verbose. Let's walk through how the engine would match the first of our regex:

ex8.regex:3

From the start of /blog/article/1...

ex8.regex:4

match /blog/article/ so that matches. Now we're on the 1 character of the corpus text.

ex8.regex:5

Match a [0-9] set, and 1 is in that set so match.

ex8.regex:6

The last thing is optional because of ? but it matched so this is matched.

ex8.regex:7

Match the end, and since were are out of characters in the corpus text we are done and this matches.

That file doesn't exist.
> ^/blog/article/[0-9]?$
Input file is empty. Use !load to load something.
>
^     # from the start
/blog/article/
[0-9] # any of 0-9
?     # optional though
$     # to the end

Input file is empty. Use !load to load something.
>