Redefined array environment can't be nested

I’m trying to redefine an "array"-like environment with xparse and it creates a lot of errors whenever I try to nest my new environment within itself:

documentclass{article}
usepackage[utf8]{inputenc}
usepackage{amsmath}
usepackage{xparse}

%%begin novalidate
NewDocumentEnvironment{newarray}{ O{rcl} +b }
{%
    begin{array}{#1}%
        #2%
    end{array}%
}{}
%%end novalidate

begin{document}

I can nest texttt{array} environments:
[begin{array}{rcl}
    1&=&left{begin{array}{rcl}
            2 &=& 3
            4&=&5
        end{array}right.
end{array}]

I can nest an texttt{array} environment within a texttt{newarray} environment:
[begin{newarray}
    1&=&left{begin{array}{rcl}
            2 &=& 3
            4&=&5
        end{array}right.
end{newarray}]

But I can't nest a texttt{newarray} environment within an texttt{array} environment:
[begin{array}{rcl}
    1&=&left{begin{newarray}
            2 &=& 3
            4&=&5
        end{newarray}right.
end{array}]

And I can't nest texttt{newarray} environments:
[begin{newarray}
    1&=&left{begin{newarray}
            2 &=& 3
            4&=&5
        end{newarray}right.
end{newarray}]

end{document}

It appears that the "&" symbols within the smaller environment are detected as new columns for the larger environment, and hence leading to many problems. Can you explain to me where this comes from? (I’m new to defining environments) Is there a way to correct this behavior? I used xparse because I want to be able to do this manipulation to the environment "IEEEeqnarraybox" of the package "IEEEtrantools", and because this environment has many optional arguments (and xparse seems well suited to defining several optional arguments). Thank you for your help!