eregon.rb, decoded by s/eval/print/

eregon.rb

# https://github.com/tric/trick2018/blob/master/honorable-mentions/eregon/entry.rb
#
# Just replacing eval by print should yield:

i,s,S=ARGF.read,[],/"(?:\\["\\\/bfnrtu]|[^"\\])*"/
while i[S]
  s << $` << $&
    i=$'
end
s << i
I=s.each{|e|e.gsub!(/\s+/,"")if e[0]!=?"}.join
G=/(?<obj>\{(?:\g<pair>(?:,\g<pair>)*)?\}){0}(?<pair>\g<str>:\g<val>){0}(?<ary>\[(?:\g<val>(?:,\g<val>)*)?\]){0}(?<val>-?(?:0|[1-9]\d*)(?:\.\d+)?(?:[eE][+-]?\d+)?|true|false|null|\g<str>|\g<ary>|\g<obj>){0}(?<str>#{S}){0}/
R=G.names.map{|n|[n,/#{G}\G\g<#{n}>/]}.to_h
M=G.to_s
C=/(\\?[^()|g]*)/
Q,H,T=/(?:\(\?:)?/,/\\g<(\w+)>/,/\)\*\)\?/
@i=0
L=->s{
  @i+=s.tr("\\","").size
  nil
}

def m(n)
  I.index(R[n],@i) or return
  a = $&
  M[/\(\?<#{n}>#{C}#{Q}#{H}#{Q}#{C}#{H}#{T}?#{C}/]
  if $2&&$2==$4
    p [1, n]
    L[$1]
    a=[]
    k=m($2)
    a<<k if k
    a<<m($2)while I[@i]==$3&&@i+=1
    L[$5]
    n=="obj" ? a.to_h : a
  elsif $2
    p [2, n]
    [*L[$1],m($2),*L[$3],m($4)]
  else M[/\(\?<#{n}>.+?\{0\}/].scan(H){|z,|z=m(z)and return z}
    p [3, n, a]
    @i+=a.size
    eval"null=nil
    #{a}"
  end
end

pp m("val")