Array alignment within align environment

Question

I am trying to typeset an array within an align environment, aligning to a specific separator in the array. In the following example, I want to align on the ":" separator in the array.

\documentclass{article}
\usepackage{amsmath, array}

\begin{document}
\begin{align*}
x^2 & = y^2 \\
z^2 &= a^2 \\
\begin{array}{l@{:}c}
a & b\\
c & d
\end{array}
\end{align*}
\end{document}

Should I be using an alignat environment for this? That usually leads to a whole bunch of extra "&"s floating around...

---

MAJOR EDIT:

Thank you to all the users who replied. I have updated my MWE with each of the solutions provided and entered my comments along with them. Basically each of the solutions solves an aspect of the problem. One solves the alignment issue and one allows me to add columns in the array environment which might not exist in the original align environment. Further comments are very welcome.

\documentclass{article}
\usepackage{amsmath, array}

\makeatletter
\newcommand{\X}[1]{\ifmeasuring@ #1\else\fgnu@X{#1}\fi}
\newcommand{\Y}[1]{\ifmeasuring@ {}:#1\else\fgnu@Y{#1}\fi}
\def\fgnu@X#1{\hbox to \ifcase1\maxcolumn@widths\fi{$\displaystyle#1$\hfil}}
\def\fgnu@Y#1{\hbox to \ifcase2\maxcolumn@widths\fi{${}:{}$\hfil$\displaystyle#1$\hfil}}
\makeatother


\begin{document}
%================================================
% original problem
\begin{align*}
x^2 & = y^2 \\
z^2 &= a^2 \\
\begin{array}{l@{:}c}
a & b\\
c & d
\end{array}
\end{align*}

%================================================
% egreg's solution
%   Cannot extend to more columns in the array than
%   in the original align environment
\begin{align*}
x^2 & = y^2 \\
z^2 &= a^2 \\
\X{aaaa} & \Y{bbbbbggggggggg}\\
\X{cccccccc} & \Y{dd}
\end{align*}

%================================================
% David Carlisle's solution
%   This solution allows me to add more columns to 
%   the array, but, the "`:"' is not centred to the
%   "`="' in the align environment.
\begin{align*}
x^2 & = y^2 \\
z^2 &= a^2 \\
\begin{array}{ll@{:}}
a &a\\
c & c 
\end{array}
&
\begin{array}{@{}cc}
b&b\\
d&d
\end{array}\\
\end{align*}

%================================================
% Barbara Beeton's solution
%   Nice alignment solution, however, can I add more
%   columns to the array?
\begin{gather*}
\begin{aligned}
x^2 & = y^2 \\
z^2 &= a^2
\end{aligned}\\
\begin{array}{l@{:}c}
a & b\\
c & d
\end{array}
\end{gather*}

Answer 1

Since you're not really using an array, I suggest another strategy:

\documentclass{article}
\usepackage{amsmath, array}
\makeatletter
\newcommand{\X}[1]{\ifmeasuring@ #1\else\fgnu@X{#1}\fi}
\newcommand{\Y}[1]{\ifmeasuring@ {}:#1\else\fgnu@Y{#1}\fi}
\def\fgnu@X#1{\hbox to \ifcase1\maxcolumn@widths\fi{$\displaystyle#1$\hfil}}
\def\fgnu@Y#1{\hbox to \ifcase2\maxcolumn@widths\fi{${}:{}$\hfil$\displaystyle#1$\hfil}}
\makeatother

\begin{document}
\begin{align*}
x^2 & = y^2 \\
z^2 &= a^2 \\
\X{aaaa} & \Y{bbbbbggggggggg}\\
\X{cccccccc} & \Y{dd}
\end{align*}
\end{document}

The left side is specified by \X{...}, the right side by \Y{...}.

Answer 2

It's not strictly necessary but easiest is to change the input order and input the two halves of the array separately. I made it a 4 column array to be slightly clearer:

\documentclass{article}
\usepackage{amsmath, array}

\begin{document}
\begin{align*}
x^2 & = y^2 \\
z^2 &= a^2 \\
\begin{array}{ll@{:}}
a &a\\
c & c 
\end{array}
&
\begin{array}{@{}cc}
b&b\\
d&d
\end{array}\\
\end{align*}
\end{document}

Answer 3

since you're using amsmath, you have other options. if i understand correctly what you want, gather* with embedded aligned and array will do it.

\documentclass{article}
\usepackage{amsmath,array}

\begin{document}
\begin{gather*}
\begin{aligned}
x^2 & = y^2 \\
z^2 &= a^2
\end{aligned}\\
\begin{array}{l@{:}c}
a & b\\
c & d
\end{array}
\end{gather*}
\end{document}

with this result:

if you have aligned material both above and below the array, getting both parts to align at the same point may be tricky. (i didn't experiment with that.)