% darcs log --tags="*" .
This is a bug! Please report it at http://bugs.darcs.net or via email to bugs@darcs.net:
Explict error in module Text.Regex.TDFA.String : Text.Regex.TDFA.String died: parseRegex for Text.Regex.TDFA.String failed:"*" (line 1, column 1):
unexpected "*"
expecting empty () or anchor ^ or $ or an atom
CallStack (from HasCallStack):
  error, called at lib/Text/Regex/TDFA/Common.hs:31:3 in regex-tdfa-1.3.2-BisVwX77VAh7AtxIZTFfgj:Text.Regex.TDFA.Common

True, the command is invalid but the message says to report so I report.

Hm, I am pretty sure we fixed that some time ago. Are you using the 
latest released version?

Indeed, I cannot reproduce it with 2.18.3, which produces the expected error message ("Invalid regular expression"). 
I should have indicated the version used, sorry.