Note that something simple as: 
sub hello();
sub hello() {print "world"}

[download]
isn't context-free (assuming the programmer is free to pick the subroutine names, and the grammer isn't going to list all possible subroutine names), and hence certainly not BNF-able.

Not even a much simpler (with respect to the language's grammar) language like C is context-free, for the same reason: forward declarations. In general, anything you have a language construct where you're forced to repeat something that wasn't matched by a terminal in the grammar (kind of like using \1 in a regex) your grammar isn't context-free - and hence, BNF will not be powerful enough.

In reply to Re: Perl not BNF-able?? by Anonymous Monk
in thread Perl not BNF-able?? by anonymized user 468275

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