Anyone know what aspects of perl should make it impossible to define it in BNF?
Perl's grammar is overall just too general for BNF.
I don't think there's an exhaustive list of all the
linguistic features in Perl that make it too general
for BNF, but I can tell you that it's not just a
couple of things, syntactically. If there's one
overall general thing to blame, it's context. To
parse Perl, you have to keep track of what context
any given bit of code that you're parsing is in.
A given syntactic construct can have a different
semantic meaning (and possibly different syntactic
implications for the surrounding code) if it is
encountered in a different context. BNF doesn't
deal well with that.
FWIW, many overly simplistic syntax highlighting
engines can't handle Perl, either, for roughly
the same reason: Perl is very expressive, and
therefore very complicated to parse. Not as
complicated (or as expressive) as, say, English,
but complicated nonetheless.
"In adjectives, with the addition of inflectional endings, a changeable long vowel (Qamets or Tsere) in an open, propretonic syllable will reduce to Vocal Shewa. This type of change occurs when the open, pretonic syllable of the masculine singular adjective becomes propretonic with the addition of inflectional endings."
— Pratico & Van Pelt, BBHG, p68
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