This is the description of the regex used previously for topic/subscription
matching. It is reproduced here for posterity.

When a message is ready to be published at the broker, we need to check all
of the subscriptions to see which ones the message should be sent to. This
would be easy without wildcards, but requires a bit more work with them.

The regex used to do the matching is of the form below for a topic of a/b/c:

^(?:(?:(a|\+)(?!$))(?:(?:/(?:(b|\+)(?!$)))(?:(?:/(?:c|\+))|/#)?|/#)?|#)$

In general, we're matching (a or +) followed by (the next levels of
hierarchy or #).
More specifically, all the levels of hierarchy must match, unless the last
level is #.

^(?:							# Must start at beginning of string
		(?:						# (Level 1 hierarchy)
			(a|\+)(?!$) 		# Match a or +, but only if not EOL.
		)						# AND 
		(?:
			(?:					# (Level 2 hierarchy)
				/				# Match /
				(?:				# AND
					(b|\+)(?!$)	# Match b or +, but only if not EOL.
				)
			)					# AND
			(?:
				(?:				# (Level 3 hierarchy)
					/			# Match /
					(?:			# AND
						c|\+	# Match c or +.
					)
				)
				|				# OR (instead of level 3)
				/#				# Match /# at level 3
			)?					# Level 3 exist 1/0 times
			|					# OR (instead of level 2)
			/#					# Match /# at level 2
		)?						# Level 2 exist 1/0 times
		|						# OR (instead of level 1)
		#						# Match # at level 1
	)$							# Must end on EOL.