The following syntaxes work as expected (they create one spoiler tag).

The general rule here is that we allow 0-3 whitespaces before the starting colon, 3 or more colons, any number of optional whitespaces, and the "spoiler" keyword with any non-whitespace character after it.

Normal

spoiler
Hello!

No whitespace

spoiler with no whitespace
Hello!

Leading whitespace

   
title
Hello!

Any number of whitespaces between : and "spoiler"

:::              spoiler title
Hello!
:::

::: spoiler title Hello! :::

Any number of whitespaces between "spoiler" and the first non-whitespace character

                title
Hello!

Three or more colons

::::::::::::::::::::::::              spoiler title
Hello!
::::::::::::::::::::::::

:::::::::::::::::::::::: spoiler title Hello! ::::::::::::::::::::::::

Here's where we run into some weird edge cases:

If there is an ending :::, Lemmy will consume the first instance of ::: only if ::: is on a new line with no other text (and up to 3 white spaces)

a
text (no whitespaces before 
)
:::

a
text (1 whitespaces before 
)
 :::

a
text (2 whitespaces before 
)
  :::

a
text (3 whitespaces before 
)
   :::

a
text (4 whitespaces before 
)
    :::

Lemmy doesn't need an ::: ending in order to display the spoiler. If ::: is not present, it will assume the spoiler continues until the end:

a

 spoiler a
text

Lemmy will create nested spoilers if it cannot find ::: before another ::: spoiler:

a
text
 spoiler b
more text

The following examples will show the ::: within the spoiler, and will make the spoilers nested

a
text (4 whitespaces before 
)
    :::

a
text

a
text 

a
text
a

Testing general functionality for creating a new post.

Edit: Testing edits again with loading indicator again

Testing another edit

HTTP link test: http://www.google.com/

Testing new post!