Commits on Mar 25, 2018

1. *: Order match regular expression alternates from longest to shortest …

   The alternation operator (|) is commutative in the POSIX spec, which
   has [1]:
   
   > If the pattern permits a variable number of matching characters and
   > thus there is more than one such sequence starting at that point,
   > the longest such sequence is matched.
   
   Testing with a few engines, here are some that match POSIX:
   
     $ echo abc | grep -Eo '|a|ab'
     ab
     $ python -c 'import regex; print(regex.match("|a|ab", "abc", regex.POSIX).group(0))'
     ab  # Background in https://bitbucket.org/mrabarnett/mrab-regex/issues/150
     $ psql -Atc "select substring('abc' from '|a|ab')"
     ab
   
   The docs for PostgreSQL are somewhat complicated, but for two or more
   branches connected by the | operator they are always greedy [2].
   
   Here are some engines that prefer the left-most branch:
   
     $ python -c 'import re; print(re.match("|a|ab", "abc").group(0))'
     $ python -c 'import regex; print(regex.match("|a|ab", "abc").group(0))'
     $ node -e 'console.log("abc".match(/|a|ab/)[0])'  # [3]
     $ ruby -e 'print "abc".match(/|a|ab/); print "\n"'  # [4]
   
   Go's stdlib provides both left-most-branch [5] and longest-match [6]
   implementations.
   
   So the old order was compliant with POSIX EREs (as referenced in
   schema/ListedLicense.xsd), but this commit sorts the branches for
   longest-match-first for compatibility with engines that break POSIX
   and prefer the left-most branch.  This only really matters when a
   longer match includes a shorter match (e.g. 'ab' includes 'a', so we
   want 'ab|a' and not 'a|ab').  It doesn't matter when the longer match
   does not include the shorter match (e.g. 'a|bc' and 'bc|a' are
   equivalent regardless of regexp engine).  But in this commit I've
   ordered by decreasing match length regardless of inclusiveness,
   because that requires less thinking ;).
   
   [1]: http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_01_02
   [2]: https://www.postgresql.org/docs/10/static/functions-matching.html#POSIX-MATCHING-RULES
   [3]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions#special-or
   [4]: https://ruby-doc.org/core-2.5.0/Regexp.html#class-Regexp-label-Alternation
        Although these docs don't have much to say on longest-match
        vs. left-most branch.
   [5]: https://golang.org/pkg/regexp/#Compile
   [6]: https://golang.org/pkg/regexp/#CompilePOSIX

   

   wking committed Mar 25, 2018
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