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#!/usr/bin/perl --
use strict; use warnings;
use File::Slurp;
...
    $ret ;
}
__END__

$ cat core.abnf
BIT            =  "0" / "1"
CR             =  %d13
...
CR             =  %b1101
CRLF           =  CR LF
LF             =  %x0A

$ perl abnf.to.grammars.pl core.abnf
<rule: BIT>  0 | 1
<rule: CR>  \15
...
<rule: CR>  \15
<rule: CRLF>  (?: <CR><LF> )
<rule: LF>  \x0A