Using parser combinators in Julia https://github.com/wdebeaum/PEG.jl

parsetree.jl

module TreeFromString

using PEG

import Base: show

struct Tree{T}
    val ::T
    children ::Vector{Tree{T}}
end

isterminal(t::Tree) = isempty(t.children)

function show(io::IO, t::Tree)
    if isterminal(t)
        print(io, t.val, ' ')
    else
        print(io, "[.", t.val, ' ')
        foreach(t -> print(io, t), t.children)
        print(io, ']')
    end
end

@rule token = r"[^\[\] ]*" & " " > 
    (terminal, space) -> Tree{String}(terminal, String[])
@rule nonterminal = "[." & r"[^\[\] ]*" & " " & tree[+] & "]" > 
    (open, nonterminal, space, children, close) -> Tree{String}(nonterminal, children)
@rule tree = nonterminal, token

let str = "[.C [.D1 D2 ] E ]"
    @assert string(tree(str)[1]) == str
end

end # module