Try:
(?<frommonth>\w\w\w)(?:_)(?<fromday>\d\d)(?:_to_)(?<tomonth>\w\w\w)(?:_)(?<today>\d\d)
Then Convert the month groups to numbers and include the date from DateTime.Now
It isn't practical to do the whole thing with a regex - it's a Text processor, not a date processor! It's a lot simpler (and more maintainable) to do the info extraction with a Regex, then process that in your preferred language.
[edit]Typo, and spurious HTML closing tags...[/edit]