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javascript match and add string in dynamic Url

I have a dynamic URL, which i want to append some string at last
'http://staging.mydomain.name.com/test/7bb12c5f7b2f4f008261bea2d3f5abd2/200x200.png'
want to append "preview" before size (200x200.png which is also dynamic), something like below
'http://staging.mydomain.name.com/test/7bb12c5f7b2f4f008261bea2d3f5abd2/preview/200x200.png'
I have seen Javascript match to remove part of file name from URL - replace the last occurence but its not what i want.
Thanks

Just use a simple regular expression like:
'http://.../200x200.png'.replace(/[^\/]+$/, 'preview/$&')

You can do like str.replace(/(/\d+x\d+.)/,'/preview$1')

Replace everything after last character in URL

I have the following code which replaces the current URL using JavaScript:
window.location.replace(window.location.href.replace(/\/?$/, '#/view-0'));
However if I have a URL like:
domain.com/#/test or domain.com/#/
It will append the #/view-0 to the current hash. What I want to is replace EVERYTHING after the last part of the URL including any query strings or hashes.
So presume my regex doesn't handle that... How can I amend it, to be more aggressive?

The following syntax may help:
location.href.replace(/[?#].*$/, '#/view')
It will replace everything after (and together with) ? or # in the string with #/view.

(^[^\/]*?\/)(?:.*)
Use this.Replace by \1 then your string
See demo.
http://regex101.com/r/sA7pZ0/28

Passing a param to param

I have the following:
html = html.replace(/[\d\.]+/g, "");
I want to the get the value of /[\d\.]+/g and put it in between the "" and then add some over jibberish after it.
Is this possible? If so how? Whats the term called of passing a value from parameter 1 to 2?

You want to reference a match from the first parameter? Fairly simple.
First, a "match" is defined inside parenthesis. This way we say "this is the first group." So, you want to match the entire string, so let's put everything between the start and end slash in parenthesis:
/([\d\.]+)/g
Now, we reference these past matches with $# where #, starting at 1, is the order in which they appear. So, our final replacement looks like this:
html = html.replace(/([\d\.]+)/g, "$1 your extra content here");
Where, as you can see, you can define your extra content.

What's wrong with this regular expression to find URLs?

I'm working on a JavaScript to extract a URL from a Google search URL, like so:
http://www.google.com/search?client=safari&rls=en&q=thisisthepartiwanttofind.org&ie=UTF-8&oe=UTF-8
Right now, my code looks like this:
var checkForURL = /[\w\d](.org)/i;
var findTheURL = checkForURL.exec(theURL);
I've ran this through a couple regex testers and it seems to work, but in practice the string I get returned looks like this:
thisisthepartiwanttofind.org,.org
So where's that trailing ,.org coming from?
I know my pattern isn't super robust but please don't suggest better patterns to use. I'd really just like advice on what in particular I did wrong with this one. Thanks!

Remove the parentheses in the regex if you do not process the .org (unlikely since it is a literal). As per #Mark comment, add a + to match one or more characters of the class [\w\d]. Also, I would escape the dot:
var checkForURL = /[\w\d]+\.org/i;

What you're actually getting is an array of 2 results, the first being the whole match, the second - the group you defined by using parens (.org).
Compare with:
/([\w\d]+)\.org/.exec('thisistheurl.org')
→ ["thisistheurl.org", "thisistheurl"]
/[\w\d]+\.org/.exec('thisistheurl.org')
→ ["thisistheurl.org"]
/([\w\d]+)(\.org)/.exec('thisistheurl.org')
→ ["thisistheurl.org", "thisistheurl", ".org"]
The result of an .exec of a JS regex is an Array of strings, the first being the whole match and the subsequent representing groups that you defined by using parens. If there are no parens in the regex, there will only be one element in this array - the whole match.

You should escape .(DOT) in (.org) regex group or it matches any character. So your regex would become:
/[\w\d]+(\.org)/
To match the url in your example you can use something like this:
https?://([0-9a-zA-Z_.?=&\-]+/?)+
or something more accurate like this (you should choose the right regex according to your needs):
^https?://([0-9a-zA-Z_\-]+\.)+(com|org|net|WhatEverYouWant)(/[0-9a-zA-Z_\-?=&.]+)$

How to match between characters but not include them in the result

Say I have a string "&something=variable&something_else=var2"
I want to match between &something= and &, so I'll write a regular expression that looks like:
/(&something=).*?(&)/
And the result of .match() will be an array:
["&something=variable&", "&something=", "&"]
I've always solved this by just replacing the start and end elements manually but is there a way to not include them in the match results at all?

You're using the wrong capturing groups. You should be using this:
/&something=(.*?)&/
This means that instead of capturing the stuff you don't want (the delimiters), you capture what you do want (the data).

You can't avoid them showing up in your match results at all, but you can change how they show up and make it more useful for you.
If you change your match pattern to /&something=(.+?)&/ then using your test string of "&something=variable&something_else=var2" the match result array is ["&something=variable&", "variable"]
The first element is always the entire match, but the second one, will be the captured portion from the parentheses, which is much more useful, generally.
I hope this helps.

If you are trying to get variable out of the string, using replace with backreferences will get you what you want:
"&something=variable&something_else=var2".replace(/^.*&something=(.*?)&.*$/, '$1')
gives you
"variable"