Empty if block with  

[kaisermann]

REPL

The template:

{#if foo}
	 
{:else}
	<p>not foo!</p>
{/if}

Gives the following warning:

[Conduitry]

Apparently /\S/ doesn't match the character that   represents. We need to be more liberal with our definition of non-whitespace characters. Perhaps [^ \n\r\t]?

[kaisermann]

According to regex101, the \S matches any non-whitespace character (equal to [^\r\n\t\f\v ]). That last empty space is the culprit...

[Rich-Harris]

fixed in 2.12.1, thanks