Problem with simple regex

Hi,

I have a problem with a substitution with a simple regex. I want to
change occurrences of page numbers and years that are using a hyphen, to
an en-dash. Something like "308-309" should become "308--309".
My best try so far is:

:s/\d-\d/\1--\2

But this creates 30--09. My understanding is that \1 and \2 keep the
value of the content of \d. Apparently not for whatever reason. What can
I do to get this replacement working?

Niels

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