Existing wordnets mainly focus on synonyms, while antonyms have often been neglected, especially in wordnets in languages other than English. In this paper, we show how regular expressions are used to generate an antonym resource for German by using Wiktionary as a source. This resource contains antonyms for 45499 words. The antonyms can be used to extend existing wordnets. We show that this is important by comparing our antonym resource to the antonyms in OdeNet, the only freely available German wordnet that contains antonyms for 3059 words. We demonstrate that antonyms are relevant for the detection of the rhetorical figure antithesis. This figure has been known to influence the audience by creating contradiction and using a parallel sentence structure combined with antonyms. We first detect parallelism with part-of-speech tags and then apply our rule-based antithesis detection algorithm to a dataset of the messenger service Telegram. We evaluate our approach and achieve a precision of 57% and a recall of 45% thus overcoming the existing approaches.