此javascript正则表达式会替换空白吗？

Question

I'm browsing through the code for twitter bootstrap and I've coma across this snippet a few times.

href.replace(/.*(?=#[^\s]+$)/, '') //strip for ie7

Regexes are my blindspot and I can't figure out most of them. What is it replacing? Is it whitespace after the #?

Side note. Can anyone recommend a good source for regex tuition?

Answer 1

Here's what it's looking for:

.*     # any number of characters (optional)
(?=    # open a lookahead
#      # find a hash tag
[^\s]+ # at least one non-whitespace character
$      # end of line
)      # close the lookahead

So, for example, it matches what comes before a hash tag:

replace this!#foobar   <-- matches: replace this!
hello world#goodbye    <-- matches: hello world
no match here !        <-- doesn't match anything because there is no hash
what?#                 <-- does not match because there is nothing after the hash
what?# asd             <-- does not match because there is a whitespace-character