I have two regex. Both matches American Date Formats. Here there are (I highlight the group I talk about):
^(.*?)((0|1)?\d)-((0|1|2|3)?\d)-**(19|20\d\d)**(.*?)$
^(.*?)((0|1)?\d)-((0|1|2|3)?\d)-**((19|20)\d\d)**(.*?)$
Both matches:
asasa12-12-1993.txt
asassa12-12-2010.txt
In the book he put 19|20 into its own group. Why?
My best guess is it's easier for humans to parse.
The first ((19|20\d\d)) doesn't make it obvious whether the alternation is "19 or 20\d\d", whereas in ((19|20)\d\d) it's obvious to see it's "19 or 20, then \d\d".
AKX is almost right but it's more than that.
19|20\d\d will match either 19 OR 20 followed by 2 digits.
But it will not match 19 followed by 2 digits.
Have a look here: https://regex101.com/r/lvYGUb/3
You'll see 2010 is a single match whereas 19 is matched alone, without the 93, and as a consequence the 93 goes with the .txt group, which is probably not what you want
In a similar way, consider this data file :
20 euros
20 €
Let's say you want to match 100% of both lines using a regex.
\d+ euros|€ won't work because it means either a number followed by the word euros OR just the € sign alone
But
\d+ (euros|€) will work
So the purpose of the parentheses here is not capturing the group, they are just meant to put a boundary to the OR operator.
If you don't want those parentheses to capture the group, you can add ?: to make it a non-capturing group, like so:
^(.*?)((0|1)?\d)-((0|1|2|3)?\d)-((?:19|20)\d\d)(.*?)$
