KSES: Prevent stripslashes from stripping escaped slashes

[sirreal]

Companion to WordPress/gutenberg#71291 and #9558.

Prevent wp_kses_stripslashes() from stripping slashes that are escaped themselves and do no escape the ".

See WordPress/gutenberg#16508 and WordPress/gutenberg#6619.

[649] introduced this slash stripping in the original version of KSES introduced in Core. It's remained the same ever since:

In trac:

wordpress-develop/wp-includes/kses.php

Lines 387 to 395 in f27aff6

	function wp_kses_stripslashes($string)
	###############################################################################
	# This function changes the character sequence \" to just "
	# It leaves all other slashes alone. It's really weird, but the quoting from
	# preg_replace(//e) seems to require this.
	###############################################################################
	{
	return preg_replace('%\\\\"%', '"', $string);
	} # function wp_kses_stripslashes

Trac ticket:

---

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[github-actions]

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[sirreal]

#9610 proposes deprecating the function. See discussion in https://core.trac.wordpress.org/ticket/63881.

[sirreal]

This is very difficult to reason about with all the levels of escaping, but I think I've confirmed that this is an issue. It's difficult to reason about additionally because the purpose of the function is not particularly clear.

$str = <<<'STR'
\"
\\"
\\\"
STR;

echo "Original:\n" . $str . "\n\n";
echo "KSES:\n" . wp_kses_stripslashes($str) . "\n\n";

Prints:

Original:
\"
\\"
\\\"

KSES:
"
\"
\\"

The interesting case is the second line. It appears to be an escaped \ followed by a " with no preceding \ (because the preceding \ was, itself, escaped).

However, \\" became \", the \" was replaced with " resulting in \". This appears to be incorrect behavior.

The first and last lines correctly replace \" with " where the preceding \ is, itself, not escaped.