Personally, I'd probably consider using the "file dirname" command since you're talking of the directory separator (/), or in a more generic case, a split/lrange/join combo.
A regular expression however, would look something like this:
set string ".this/is/some/text/in/here"
set pattern {^(.*)/[^^]+$}
regexp -- $pattern $string Match subMatch1
#You'll now find "./this/is/some/text/in" in the $subMatch1 variable, if we've got a match...