How to align the middle of some words

Question

I want to align the middle of a "word" with something else. Let me explain. The following is what I have.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
a&:=b\\
c&=d
\end{align*}
\end{document}

This looks okay, but I really want the two equal signs to be what we are aligning. So the obvious thing to do is to more the & to the other side of the :, as follows.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
a:&=b\\
c&=d
\end{align*}
\end{document}

But, of course, this was never going to work! You end up with a space between the colon and the equals sign, : = as opposed to :=. So you see my problem...

Answer 1

This is using the under-development tabstackengine package, first introduced here at Writing a table with equally spaced columns, based on the widest column (source code available at Measuring align).

The package extends the stackengine package by adding tabbing capability. This answer, Can I tab inside of align environment?, gives some of the syntax of the package. I apologize that I have been lax in completing the package and getting it out the door, so the only way to see it in action is by searching this site for tabstackengine.

---

\documentclass{article}
\usepackage{tabstackengine}
\usepackage{amsmath}
\stackMath
\begin{document}
\begin{equation*}
\setstacktabulargap{0pt}
\tabularCenterstack{lrl}{%
a&:=&b\\
c&=&d
}
\end{equation*}
\end{document}

Answer 2

Is the following what you are looking for?

\documentclass{article}

\usepackage{mathtools}

\begin{document}

\begin{align*}
  a \coloneqq{} &b\\
  c ={}         &d
\end{align*}

\end{document}

Maybe

\documentclass{article}

\usepackage{mathtools}

\begin{document}

\begin{alignat*}{2}
  &a &{}\coloneqq{} &b\\
  &c &{}={}         &d
\end{alignat*}

\end{document}

is (more) useful?

Note: \coloneqq from mathtools gives the correct symbol for dynamic definition (or whatever it is).

Answer 3

If a bigger space in the second equation is acceptable, the following is working quite easy. In this very case, the right side aligning of the left terms actually looks quite nice.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{align*}
a&:=b\\
c&\phantom{:}=d
\end{align*}
\end{document}

For the use of the better looking \coloneqq of the package mathtools, just align on the right side of the equations: ={}&