Fixed exclusion of certain named groups in $Matches.

GetGroupNames returns numeric strings for every unnamed group but at least
PowerShell does not include those (only named groups as strings and numbered
groups as integers). I mistakenly excluded only "0" from the group names, not
noticing that every group is present there, not only the full match. Now the
exclusion criterion is "starts with a digit", although the documentation does
not say whether a group may not start with 09 or whether every digit is
disallowed. I hope for the latter with this implementation, although real-world
impact is likely very low, even if wrong.