Looks like it is searching for a string that starts with a digit (followed by 3 more digits), then 2 more digits, and ending with 2 more digits (8 digits total).

It is then grouping them as the first 4 digits in one group (likely to specify a year), followed by a group of 2 digits (likely month), followed by the last group of 2 digits (likely day).

The first index of the array is the entire match, followed by each group: 4-digits then 2-digits then 2-digits. I would expect a 4 element array.

This indicates the expected pattern for this Regular Expression is in the form 00009944

As you stated, that would most likely be a date (particularly since the source variable is named ‘expires’!)

Note, also the caret at the beginning and the dollar sign at the end (as @cpradio; provided it) forces a match of EXACTLY 8 digits and nothing else.
In other words, if you were to use this Regular Expression to evaluate a string like “I will meet you on 20130415 at dusk.” it would not match it.

It would have to be 8 numbers/digits (as @ParkinT ; also mentioned), ideally in a specific format yyyyMMdd, for example. So the following would all be valid:
20130806
20000714
20181215
etc.

Things that are invalid
2013-08-06
2000/07/14
2018.12.15