^(19|20)?[0-9]{6}[- ]?[0-9]{4}$ I know that ^ and $ tells us the beginning and end of the string. [0-9]{6} and [0-9]{4} tells us to have numbers from 0-9 six times and four times. [- ]? tells us that we can have "-" between them. But why do we have space between - and ]. and to the question that I'm wondering the must, what does "(19|20)?" do. I know that | is an or, so 19 or 20. But I can still use another number like 60 or whatever at the beginning without getting any error. What is the benefit of (19|20)? :)

Can someone explain this regex, especially what "(19|20)?" does in ^(19|20)?[0-9]{6}[- ]?[0-9]{4}$

^(19|20)?[0-9]{6}[- ]?[0-9]{4}$

I know that ^ and $ tells us the beginning and end of the string.
[0-9]{6} and [0-9]{4} tells us to have numbers from 0-9 six times and four times.
[- ]? tells us that we can have "-" between them. But why do we have space between - and ].

and to the question that I'm wondering the must, what does "(19|20)?" do. I know that | is an or, so 19 or 20. But I can still use another number like 60 or whatever at the beginning without getting any error. What is the benefit of (19|20)? :)

Can someone explain this regex, especially what "(19|20)?" does in ^(19|20)?[0-9]{6}[- ]?[0-9]{4}$

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