A rule-based Korean dialect converter with no LLM: eight guards that keep standard Korean intact

I built a Korean dialect converter that runs entirely in the browser and uses no LLM. Type a standard Korean sentence and it rewrites it the way people speak in Gyeongsang, Jeolla, Chungcheong, Gangwon or Jeju. Paste a dialect sentence and it converts back to standard Korean. Nothing leaves the browser. Try it: https://toolnjoy.com/dialect-converter (Korean UI) The hard part was not producing dialect. It was not breaking standard Korean . Every time I widened a rule, a perfectly normal sentence got mangled somewhere. Here is what I ended up with. Most of it applies to any rule-based text rewriter (honorific converters, profanity filters, spelling normalizers). Shape of the thing Four modules: file job core sentence splitting, sentence-type detection, ending replacement endings per-region ending tables (declarative / interrogative / propositive / imperative × politeness) words per-region vocabulary reverse dialect → standard Forward conversion goes "decide the sentence type, then pick the region's ending". Reverse goes the other way: "look at the ending, infer the sentence type". You cannot just invert the forward table, so reverse has its own table. Each reverse rule carries a type , which removes the detection step entirely. In reverse mode the user does not pick a region. I run all five and keep the one with the highest score (4 points per matching ending + syllable count of matched words). People who paste dialect usually do not know which region it is from. That is why they came. Eight guards, each from a real failure 1. Never reverse a form that appears on the standard side of any table. The Gyeongsang table had 함께 → 같이 (together). Reverse flipped it and rewrote "같이 보게" into "함께 보게", except 같이 is perfectly standard. Fix: anything that ever appears in a standard column anywhere is excluded from reverse candidates. 2. Question endings only count when there is a question mark or a wh-word. Gyeongsang -나 / -노 mark questions. But 하나 ("one") also ends in -나 . Without