import re
s = "Add.1, 2020 and Add.1, 2021-2023, 2025, 2028 and 2029 and Add.1) R"
r = "(?:\s|,|and|Add\S*?|Parts?|\([^\)]*\)|[IV\-\d]+)*$"
print "It's going to crash"
print re.search(r, s)
print "It hasn't crashed"

I assume this is one of the pathological cases where the RE algorithm backtracks "to death". That's not really a bug, but a limitation.

I can shorten it further to: 

s = "Add.1, 2020 Add.1, 20212023, 2025, 2028 and 2029 Add.1) R"
r = "(?:\s|,|and|Add\S*?|[\d]+)*$"

Sometimes it finishes after a long time when you shorten the s-string from the front.  If you take the "R" from the end it goes through straight away.  It's even very slow with match.  

Tested on 2.6rc2 and slow but successful.  Issue 1662851 may be related.

This problem has been addressed in issue #2636.

Although the extra checks certainly aren't foolproof, neither of the
examples given are slow.

If at all, this will be fixed by applying #2636.