functions fmtmatch() and fmtre() to convert between glob and ERE patterns are buggy

[krader1961]

I recently added many more API tests of the fmtre() and fmtmatch() functions. Primarily to see if I could improve test coverage to acceptable levels. However, the API tests don't actually verify that the patterns are equivalent; only that the conversion produces the expected, but possibly incorrect, output. So I just added (not yet commited) tests to src/cmd/ksh93/tests/sh_match.sh to verify the patterns produce the same results when used in a shell script. Not surprisingly some patterns are incorrectly converted:

<E> sh_match[450]: pattern #13 ere |^x\!y$| and glob |x\!y| produce diff output
<E> sh_match[450]: pattern #20 ere |^x|y$| and glob |@(x|y)| produce diff output
<E> sh_match[450]: pattern #31 ere |^x\a|b$| and glob |@(xa|b)| produce diff output
<E> sh_match[493]: pattern #13 ere |^x\!y$| and glob |x!y| produce diff output
<E> sh_match[493]: pattern #21 ere |^x|y$| and glob |x|y| produce diff output
<E> sh_match[493]: pattern #34 ere |^z.?a$| and glob |~(E)z.?a| produce diff output
<E> sh_match[493]: pattern #38 ere |^x\a|b$| and glob |x\a|b| produce diff output

Notice that most of the failures involve patterns that use the alternation, |, operator. Consider the ERE ^x|y$. That means match lines that begin with x or end with y. But the glob produced by fmtmatch() is @(x|y). That means match lines that are one char in length where that char is x or y. In other words the generated glob is actually equivalent to the ERE ^(x|y)$.

These functions are cool but I would never trust their output and therefore would never use them. If we were evaluating whether they should be added to the project I would have voted no. Sadly, even though it is highly unlikely any ksh script is using this feature we can't afford to break backward compatibility by removing it.

[krader1961]

P.S., The way to use these in a script is to do something like this:

glob=$(print -f '%P' -- "$ere")
[[ "$line" = $glob ]] && print matches
ere=$(print -f '%R' -- "$glob")
[[ "$line" =~ $ere ]] && print matches