[oak perl] Backslash Space in Regex?

[Eugene]

Hi All,
In a Perl regular expression,
what does backslash space mean?   <<<<<
Eugene

Why I'm Asking (in case you care)
--------------------
I was reading a book that is definitely not a Perl book,
but the book uses Perl a lot 
and contains a very short introduction to Perl.

In the regex part of the introduction,
the author has an example in which the regex contains both
* \s 
and 
* backslash space
In his overall explanation of the regex,
he doesn't distinguish between the two.
In his explanation of some of the elements of the regex, 
he explains \s but doesn't explain backslash space .

I have never seen backslash space in a regex before,
so I'm wondering what it means exactly.