simple regexp question

jeanluca has asked for the wisdom of the Perl Monks concerning the following question:

Dear Monks,

I need to parse dates. For example the following to dates are possible:

2005-100
2005100
[download]

So the only difference is that there might be an - or not. I'm not sure how to create an 'or' in a regexp, so what should my regexp look like ?

Thanks a lot
Luca

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