provides details of failed regex compilation in the error message we throw in Regex-constructor (it's good to know what exactly is wrong)

fail2ban/server/failregex.py

@@ -145,9 +145,9 @@ class Regex:
 				n = ALTNAME_CRE.match(k).group(1)
 				self._altValues[k] = n
 			self._altValues = list(self._altValues.items()) if len(self._altValues) else None
-		except sre_constants.error:
-			raise RegexException("Unable to compile regular expression '%s'" %
-								 regex)
+		except sre_constants.error as e:
+			raise RegexException("Unable to compile regular expression '%s':\n%s" %
+								 (regex, e))
 		# set fetch handler depending on presence of alternate tags:
 		self.getGroups = self._getGroupsWithAlt if self._altValues else self._getGroups
 

fail2ban/tests/fail2banregextestcase.py

@@ -133,6 +133,13 @@ class Fail2banRegexTest(LogCaptureTestCase):
 			"test", r".** from <HOST>$"
 		))
 		self.assertLogged("Unable to compile regular expression")
+		self.assertLogged("multiple repeat", "at position 2", all=False); # details of failed compilation
+		self.pruneLog()
+		self.assertFalse(_test_exec(
+			"test", r"^(?:(?P<type>A)|B)? (?(typo)...) from <ADDR>"
+		))
+		self.assertLogged("Unable to compile regular expression")
+		self.assertLogged("unknown group name: 'typo'", "at position 23", all=False); # details of failed compilation
 
 	def testWrongIngnoreRE(self):
 		self.assertFalse(_test_exec(
@@ -140,6 +147,7 @@ class Fail2banRegexTest(LogCaptureTestCase):
 			"test", r".*? from <HOST>$", r".**"
 		))
 		self.assertLogged("Unable to compile regular expression")
+		self.assertLogged("multiple repeat", "at position 2", all=False); # details of failed compilation
 
 	def testWrongFilterOptions(self):
 		self.assertFalse(_test_exec(