My Weird Prompts
Daniel Rosehill
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One Regex Rule Worked, One Was Impossible
A listener wrote one text-replacement rule in an Android dictation app and it erased every "um" and "uh" from his transcripts. Then he wrote a second rule about capitalization, and it failed — not because it was fiddly, but because it's provably impossible. This episode uses that gap as a case study in what regular expressions actually are: a 1951 notation from Stephen Cole Kleene, first implemented by Ken Thompson in 1968, now a family of dialects (POSIX, Perl, PCRE2, RE2) that disagree about what the word even means. Along the way: why the app's Accessibility-service architecture constrains everything you can write on Android, why JavaScript's own reference admits its regexes aren't regular, and why LLMs haven't made any of this obsolete. Key sources: • Whisperian official site, Issue N° 801, dated 2026-03-12. — https://whisperian.app • Android Pattern API reference. — https://developer.android.com/reference/java/util/regex/Pattern • Android libcore Pattern source (ICU-backed). — https://android.googlesource.com/platform/libcore/+/refs/heads/master/ojluni/src/main/java/java/util/regex/Pattern.java Episode 5938 — myweirdprompts.com/n/5938 (permanent short link: myweirdprompts.com/294517)
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