What's the exact syntax for spoiler markup [duplicate]

Question

Following the Do we want the “spoiler” feature turned off? thread on meta.tex.sx, it's clear that the spoiler markup has a wrinkle or two that I didn't at first appreciate. For information's sake, the spoiler feature is contentious because it collides with the syntax for Latex errors.

Is there a precise description of when a blockquote will be treated as a spoiler?

Two related threads containing some information on the syntax:

1. Implement the <spoiler> tag
2. Add markdown support for hidden-until-you-click text (aka spoilers)

Answer 1

The way spoiler works is as follows.

If each line in a blockquote (built via >) begins with a ! it will be treated as a spoiler.

So,

>! This is a spoiler

This is a spoiler

>! But this is  
> not a spoiler

! But this is
not a spoiler

>! A multi-line spoiler  
>! has ! multiple times

A multi-line spoiler
has ! multiple times

>! Alignment  
> ! has no effect  
 >  ! on spoilers  

Alignment
has no effect
on spoilers

Due to wrinkle in how blockquotes are generated, leading white space is pretty pointless.

> ! <- still a spoiler

<- still a spoiler

The actual code expects very specific html (this a "post markdown" process). In particular, omitting the inner <p> will dodge "spoiler-ification".

The only way to guarantee a blockquote becomes a spoiler is to use the explicit >! syntax.

---

Prior to implementing this, I did actually check for posts that would be affected by this syntax (on SO its under one-tenth of a percent*). Even on TeX, (where ! is rather likely to come up as a first character) it is exceedingly rare (lower than SO levels). People seem very biased towards using code tags.

I'm not saying there aren't any, but any new syntax is going to catch somebody.

_{*Admittedly, I'm regex-ing wildly through lots of data, there is some room for error. Pretty sure I'm erroring on the side of over counting affected posts, though.}