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Split string with brace to become parameter

If I have a input value "a[123],b[456],c[789]" and I want to return as "a=123&b=456&c789"
I've tried below code but no luck.. Is there a correct way to implement this?
var str = "a[123],b[456],c[789]"
var string = (str).split(/\[|,|\]/);
alert(string);

One option is:
var rep = { '[': '=', ']': '', ',': '&' };
var query = str.replace(/[[,\]]/g, el => rep[el] );

The delimiters are already there, it's just a matter of replacing one delimiter with another. Replace each [ with an =, replace each , with an &, and remove all ].
var str = "a[123],b[456],c[789]"
var string = str.replace(/([a-z])\[(\d+)],?/g, '$1=$2&').slice(0, -1);
alert(string);

Brute force way im not good at Regex. Just adding my thoughts
var str = "a[123],b[456],c[789]"
str = str.replace(/],/g, '&');
str = str.replace(/\[/g, '=');
str = str.replace(/]/g,'');
alert(str);

The simple 2 line answer for this is:
str=str.replace(/,/g,"&");
str=str.replace(/(\w)\[(\d+)\]/g,"$1=$2");

Replace multiple occurences of a comment with javascript

I want to replace multiple occurences of comment and try like below
JsFiddle
Code:
var str = '<!--#test--><!--#test1-->'
str = str.replace('<!--/g', '').replace('-->/g', '');
alert(str)

Your problem is that you're trying to use a string instead of a regular expression. For example, this works.
var str = '<!--#test-->'
str = str.replace(/<!--/g, '').replace(/-->/g, '');
alert(str)

Plain regex commands need to be inside //.
Also, use the
Disjunction; Alternative | (pipe character)
str = str.replace(/<!--|-->/g, ''); // #test#test1

Get string piece after last symbols "/" or "\"

I need get from string, piece after last \ or /, that is from this string C:\fake\path\some.jpg result must be some.jpg
I tried this:
var str = "C:\fake\path\some.jpg";
var newstr = str.replace(/[^\\\/]+$/, "");
alert(newstr);
http://jsfiddle.net/J4GdN/3/
but not works, what is right regex for this?

You don't need a regex to do it, this should work:
var newstr = str.substring(Math.max(str.lastIndexOf("\\"), str.lastIndexOf("/")) + 1);

Well your str needs to be escaped \\ for it to work correctly. You also need to use match since the reg exp you are using is matching the end, not the start.
var str = "C:\\fake\\p\ath\some.jpg";
var newstr = str.match(/[^\\/]+$/);
alert(newstr);
JSFiddle

first, let's escape the backslashes
var str = "C:\\fake\\path\\some.jpg";
var newstr = str.replace(/[^\\\/]+$/, "");
alert(newstr);
now, you are removing some.jpg instead of getting it as the result.
here are a few options to fix this:
1.replace with a regex
var newstr = str.replace(/.*[\\\/]/, "");
2.split with regex (doesn't work in all browsers)
var tmp = str.split(/\\|\//);
var newstr = tmp[tmp.length-1];
4.as said here, match
var newstr = str.match(/.*[\\\/](.*)/)[1];

Replace a string of the last 7 chars?

This is my code :
​var myStr = "/private_images/last-edit/image-work-med.png";​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​
and I'd like to replace the last 7 chars (med.png) with big.png. Or, as you can see, the last occurence after a - split.
How can I do it? I think about regex, but I'm not a champion with them. Tried :
myStr = myStr .replace(/-([^-]*)$/, "big" + '$1');
but it replace the last -, not the last occurence. So the result is /private_images/last-edit/image-workbigmed.png

I'll make a confession: I'm not so great with regexes either.
How about splitting up using split? Less concise, but easier to understand.
var myStr = "/private_images/last-edit/image-work-med.png";​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​
var strs = myStr.split('-');
// Change the last element.
strs[strs.length - 1] = "big.png";
// And put back the right string.
myStr = strs.join('-');

You could use a regex, or you could use a few string methods and make your intentions clear.
var idx = myStr.lastIndexOf("-");
var newStr = myStr.substring(0, idx) + "big.png";

Without using RegExp you could use:
var str = "/private_images/last-edit/image-work-med.png"
,replace = 'big.png'
,nwstr = str.slice(0,str.lastIndexOf('-')+1)+replace;
//=> nwstr now "/private_images/last-edit/image-work-big.png"
More 'functional':
var nwstr = function(s){
return s.replace(s.substr(-7),'');}(
'/private_images/last-edit/image-work-med.png'
)+'big.png'

var url = "/private_images/last-edit/image-work-med.png";
var index = url.lastIndexOf('-');
url = url.substring(0, index+1);
var url2 = "big.png";
var output = url.concat(url2); alert(output);
Check this

Just add '-' to your regex and to the replacement string:
myStr = myStr .replace(/-([^-]*)\.png$/, "-big.png");
Or if you want the file extension to be variable:
myStr = myStr .replace(/-([^-]*)\.([a-z]+)$/, "-big.$2");

Why not just use replace:
var myStr = "/private_images/last-edit/image-work-med.png";​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​
var newStr = myStr.replace("med.png", "big.png");
According to the requirements specified in your question this would suffice.

If you know it will be a .png file:
var ex = new Regex(#"-\w*.png$");
var myStr = "/private_images/last-edit/image-work-med.png";​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​
myStr = ex.Replace(myStr, "-big.png");
It works but if its a jpg it wont...

If you want to use string functions -
var myStr = "/private_images/last-edit/image-work-med.png";
var cleanedupStr = myStr.slice(0, myStr.lastIndexOf("-"));
String.slice

Regular Expression in JS

How do you trim all of the text after a comma using JS?
I have: string = Doyletown, PA
I want: string = Doyletown

var str = 'Doyletown, PA';
var newstr=str.substring(0,str.indexOf(',')) || str;
I added the || str to handle a scenario where the string has no comma

How about a split:
var string = 'Doyletown, PA';
var parts = string.split(',');
if (parts.length > 0) {
var result = parts[0];
alert(result); // alerts Doyletown
}

using regular expression it will be like:
var str = "Doyletown, PA"
var matches = str.match(/^([^,]+)/);
alert(matches[1]);
jsFiddle
btw: I would also prefer .split() method

Or more generally (getting all the words in a comma separated list):
//Gets all the words/sentences in a comma separated list and trims these words/sentences to get rid of outer spaces and other whitespace.
var matches = str.match(/[^,\s]+[^,]*[^,\s]+/g);

Try this:
str = str.replace(/,.*/, '');
Or play with this jsfiddle