[TUHS] Re: A fuzzy awk

[Douglas McIlroy <douglas.mcilroy@dartmouth.edu>]

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> Doug McIlroy was generating random regular expressions

Actually not. I exhaustively (within limits) tested an RE recognizer
without knowingly generating any RE either mechanically or by hand.

The trick: From recursive equations (easily derived from the grammar of
REs), I counted how many REs exist up to various limits on token counts,
Then I generated all strings that satisfied those limits, turned the
recognizer loose on them and counted how many it accepted. Any disagreement
of counts revealed the existence (but not any symptom) of bugs.

Unlike most diagnostic techniques, this scheme produces a certificate of
(very high odds on) correctness over a representative subdomain. The scheme
also agnostically checks behavior on bad inputs as well as good.  It does
not, however, provide a stress test of a recognizer's capacity limits. And
its exponential nature limits its applicability to rather small domains.
(REs have only 5 distinct kinds of token.)

Doug

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