Actually? In this case I prefer the regexen without the x modifier. The regex
/^\s*$/ is not long enough to require internal comments, the comment after the regex explains it fully. The addition of whitespace just spreads it out.
As opposed, of course, to potentially confusing regexen such as /\[\s*([^\]]+)/ which, I think, gained a lot of readability with the x:
/\[\s* # literal [ followed by optional whitespace
( # begin capture
[^\]]+ # get one or more chars that are not ]
) # end capture
/x # end regex
(note that this regex is part of a script at work where the future maintainers will probably not know regexen well, which explains the over commenting)