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How to use a variable in a match method with regular expression pattern

I want to get an array in JavaScript from a string, cutting it in half.
Example:
// From this:
var myStr = 'cocacola';
// To this:
var myArray = ['coca', 'cola'];
I tried the following method:
var myStr = 'cocacola';
var strHalf = myStr.length / 2;
// This won't work
var myArray = myStr.match(/.{1,strHalf}/g);
// Only this will work fine
var myArray = myStr.match(/.{1,4}/g);

You can do String.slice() to solve this
var myStr = 'cocacola';
let len = myStr.length;
let result = [myStr.slice(0, len/2), myStr.slice(len/2)]
console.log(result);

You'll need to to use the RegExp() constructor and make a string that it understands:
var regexString = "/.{1," + strHalf + "}/g";
var myRegex = new RegExp( regexString );
var myArray = myStr.match( myRegex );
...but be careful doing this; if strHalf is a string containing special characters like / then your RegExp will have weird behaviour.

You can create a non-hardcoded regexp by using the RegExp constructor:
var myArray = myStr.match(new RegExp('/.{1,'+strHalf+'}/g'));
The constructor has no clue that strHalf should be an integer, so make sure you can control that.
This is known to introduce a lot of security issues, please don't use this in production. Regexes are too often used when they shouldn't. If you ever use a regex, do look into other options
There are much better alternatives, but at least it's possible!

You dont need regex for that.
The easiest way is that:
var myStr = 'cocacola';
var strHalf = myStr.length / 2;
var array = [myStr.substr(0, strHalf),myStr.substr(strHalf, myStr.length)];
jsfiddle: https://jsfiddle.net/cudg8qc3/

You can just slice the string and place the first element in the first slot, and the second element in the second slot in the array.
var str_len = myStr.length
var my_array = [myStr.slice(0, str_len/2), myStr.slice(str_len/2, str_len)]

Passing Parameter into function match

I am using the function match for a search engine, so whenever a user types a search-string I take that string and use the match function on an array containing country names, but it doesn't seem to work.
For example if I do :
var string = "algeria";
var res = string.match(/alge/g); //alge is what the user would have typed in the search bar
alert(res);
I get a string res = "alge": //thus verifying that alge exists in algeria
But if I do this, it returns null, why? and how can I make it work?
var regex = "/alge/g";
var string = "algeria";
var res = string.match(regex);
alert(res);

To make a regex from a string, you need to create a RegExp object:
var regex = new RegExp("alge", "g");
(Beware that unless your users will be typing actual regular expressions, you'll need to escape any characters that have special meaning within regular expressions - see Is there a RegExp.escape function in Javascript? for ways to do this.)

You don't need quotes around the regex:
var regex = /alge/g;

Remove the quotes around the regex.
var regex = /alge/g;
var string = "algeria";
var res = string.match(regex);
alert(res);

found the answer, the match function takes a regex object so have to do
var regex = new RegExp(string, "g");
var res = text.match(regex);
This works fine

Javascript regexp does not work

Test string is "page-42440233_45778105"
pattern "(page-\d+_\d+)"
Online tester(http://www.regexr.com/) successfuly finded mathc,but in browser js result is null. Why?
var re = new RegExp("(page-\d+_\d+)", "gim");
var r_array = message.match(re);
console.log(r_array);

I think this would be a better pattern
var re = /^page-\d+_\d+$/i;
It also matches the beginning (^) and end ($) of the string
message.match(re);
//=> ["page-42440233_45778105"]

You need to escape \ if you use string literal:
var message = "page-42440233_45778105";
var re = new RegExp("(page-\\d+_\\d+)", "gim");
var r_array = message.match(re);
console.log(r_array);
// => ["page-42440233_45778105"]
More preferably, use regular expression literal:
var re = /(page-\d+_\d+)/gim;

When you use a string literal, you must escape the \ :
var re = new RegExp("(page-\\d+_\\d+)", "gim");
A better solution here would be to use a regex literal :
var re = /(page-\d+_\d+)/gim
Don't use the RegExp constructor if the regular expression is constant, regex literals are much more convenient.

Regex working fine in C# but not in Javascript

I have the following javascript code:
var markdown = "I have \(x=1\) and \(y=2\) and even \[z=3\]"
var latexRegex = new RegExp("\\\[.*\\\]|\\\(.*\\\)");
var matches = latexRegex.exec(markdown);
alert(matches[0]);
matches has only matches[0] = "x=1 and y=2" and should be:
matches[0] = "\(x=1\)"
matches[1] = "\(y=2\)"
matches[2] = "\[z=3\]"
But this regex works fine in C#.
Any idea why this happens?
Thank You,
Miguel

Specify g flag to match multiple times.
Use String.match instead of RegExp.exec.
Using regular expression literal (/.../), you don't need to escape \.
* matches greedily. Use non-greedy version: *?
var markdown = "I have \(x=1\) and \(y=2\) and even \[z=3\]"
var latexRegex = /\[.*?\]|\(.*?\)/g;
var matches = markdown.match(latexRegex);
matches // => ["(x=1)", "(y=2)", "[z=3]"]

Try non-greedy: \\\[.*?\\\]|\\\(.*?\\\). You need to also use a loop if using the .exec() method like so:
var res, matches = [], string = 'I have \(x=1\) and \(y=2\) and even \[z=3\]';
var exp = new RegExp('\\\[.*?\\\]|\\\(.*?\\\)', 'g');
while (res = exp.exec(string)) {
matches.push(res[0]);
}
console.log(matches);

Try using the match function instead of the exec function. exec only returns the first string it finds, match returns them all, if the global flag is set.
var markdown = "I have \(x=1\) and \(y=2\) and even \[z=3\]";
var latexRegex = new RegExp("\\\[.*\\\]|\\\(.*\\\)", "g");
var matches = markdown.match(latexRegex);
alert(matches[0]);
alert(matches[1]);
If you don't want to get \(x=1\) and \(y=2\) as a match, you will need to use non-greedy operators (*?) instead of greedy operators (*). Your RegExp will become:
var latexRegex = new RegExp("\\\[.*?\\\]|\\\(.*?\\\)");

Simple way to use variables in regex

This almost has the answer...
How do you use a variable in a regular expression?
I need to know if I can use variables instead of hardcode in the regex?
str1 = str1.replace(/abcdef/g, "stuvwxyz");
can I use variables instead of /abcdef/g and "stuvwxyz"

Of course that you can, every single bit of this can be dynamic:
var pattern = 'abcdef';
var input = 'stuvwxyz';
var modifiers = 'g';
var regex = new RegExp(pattern, modifiers);
var str1 = 'Hello abcdef';
str1 = str1.replace(regex, input);
Checkout the docs as well.

Yes.
var pattern = /abcdef/g;
var input = "stuvwxyz";
str1 = str1.replace(pattern, input);

Like this?
var regex = /abcdef/g;
var string = "stuvwxyz";
var str1 = "abcdef";
str1 = str1.replace(regex, string);