Add another hidden requirement of dateparser

Author: mhucka

requirements.txt

@@ -34,8 +34,9 @@ rich                     >= 10.1.0
 setuptools               >= 57.0.0
 sidetrack                >= 1.4.0
 
-# Handprint doesn't use six, but it's used by dateparser, and apparently
-# installation of dateparser doesn't cause installation of six.
+# Handprint doesn't use six or regex, but they're used by dateparser, and
+# apparently installation of dateparser doesn't cause their installation.
+regex                    >= regex-2021.4.4
 six                      >= 1.16.0
 
 StringDist               >= 1.0.9