That would be the /x
modifier:
x (PCRE_EXTENDED)
If this modifier is set, whitespace data characters in the pattern are totally ignored except when escaped or inside a character class, and characters between an unescaped # outside a character class and the next newline character, inclusive, are also ignored. This is equivalent to Perl's /x modifier, and makes it possible to include comments inside complicated patterns. Note, however, that this applies only to data characters. Whitespace characters may never appear within special character sequences in a pattern, for example within the sequence (?( which introduces a conditional subpattern.
It also allows commenting the pattern, which is extremely useful:
{<div\s+class=\"article\"> # many spaces between the div and the attribute
<h2(.*) # don't really care about closing the tag
</div>}sx