Does this javascript regex replace whitespace?

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?

Side Answer: regular-expressions.info

musefan
– musefan

2012-04-18 14:04:40 +00:00
Commented Apr 18, 2012 at 14:04 — musefan
– musefan, Commented Apr 18, 2012 at 14:04

Christoph · Accepted Answer · 2012-04-18 15:00:58Z

6

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

edited Apr 18, 2012 at 15:00

Christoph

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answered Apr 18, 2012 at 14:07

alan

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6 Comments

musefan Over a year ago

a few examples of valid and invalid matches would go nicely with that answer ;)

James South Over a year ago

@alan: Absolutely brilliant! This is the kind of clear answer I was hoping for.

Christoph Over a year ago

@JamesSouth to be exact: it matches #foobar and #goodbye, not replace this! and hello world. It doesn't match #f oobar or # though.

alan Over a year ago

@Christoph: No. The hash and everything after it is enclosed in a positive lookahead, which does not participate in the match. The only thing that matches the regex is what is before the lookahead.

Christoph Over a year ago

sry, i confused the word match. what i really ment: the replace returns you the #foobar and #goodbye because it matches the rest of the string and replaces it with ''. Some kind of double negation... My fault;)

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