@drj I'm not sure if that's true! Kleene's 1951 paper is sort of weird (it's all about neural models) but definitely seems to use & and /, not |. https://web.archive.org/web/20201007183213/https://www.rand.org/content/dam/rand/pubs/research_memoranda/2008/RM704.pdf. What is the earliest paper on regular expressions that switched to vertical lines?
@rk pointed out that BNF uses | for alternation so that might be a point along the way for both uses, or the influence could go the other way if there is a regular language tradition post-1951 that started using |.
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