How to read Web pages with funny backslashes?

  I'm totally out of the loop with what the young'uns are up
  to on the interwebs these days. The other day I stumbled
  across this webpage that said:

|Many textbooks, for instance, will tell you that the metric
|tensor of special relativity takes a form like
|
|\[ \eta_{\mu\nu}=\begin{pmatrix}-1&0&0&0\0&1&0&0\0&0&1&0 . . .

  How am I supposed to make heads or tails of this? I figured
  it was meant to be LaTeX, so I tried to copy it into LaTeX as,

|\documentclass[a4paper]{article}
|\begin{document}
|Here's something that caused me undue confusion over the years . . .
|Many textbooks, for instance, will tell you that the metric . . .
|\[ \eta_{\mu\nu}=\begin{pmatrix}-1&0&0&0\0&1&0&0\0&0&1&0\ . . .

  , but I got hit with:

|! Misplaced alignment tab character &.
|l.12 \[ \eta_{\mu\nu}=\begin{pmatrix}-1&
|                                      0&0&0\0&1&0&0\0&0&1&0\0& . . .
|
|?

  .