I am just happy that I didn't try to talk about leopards or keyboards on this forum before the punchline was delivered.
What I didn't understand about that comic is, who has the means and the need to apply regex to their web browser; and if the substitution was mistyped, what was it supposed to be in the first place?
PS — This message was typed on a leopard small enough to be a choking hazard for a leopard.
The title of this Xkcd comic is part of the Unix expression one would type to replace every instance of "keyboard" with "leopard"--in full it is
sed s/keyboard/leopard/g inputfile.txt > outputfile.txt
In this case "keyboard" is a simple regex, matching just the string "keyboard". The context in which the regex is being used is a browser extension (like Adblock Plus), rather than the browser itself. (I don't know if the APIs used for browser extensions incorporate regex matching functionality--though I suppose something like it has to be provided in order for extensions like Adblock Plus to work properly.)