Ufdjrw

New to coding. I don't understand the principle of [match].
I mean;

 var wikipediaDontRedirectRegExp = new RegExp("^https?://([a-zA-Z0-9\-_]+)\.(?:m\.)?wikipedia\.(?:com|org)/wiki/(.*)(?:\?oldformat=true|\?previous=yes)", "i");
match = requestedURL.match(wikipediaDontRedirectRegExp);
if (match && match.length === 3) 
 lang = match[1];
 page = match[2];
 returnVal.redirectURL = 'http://' + getFullHostName() + '/' + lang + '/' + page;
 returnVal.domain = 'imgur';
 returnVal.autoRedirect = false;
 returnVal.lang = lang;
 return returnVal;

On this code, I don't understand that match[1] or match[2] refers.
For example, on

"^https?://([a-zA-Z0-9-_]+).(?:m.)?wikipedia.(?:com|org)/wiki/(.*)(?:?oldformat=true|?previous=yes)",
"i"

what is match[1], what is match[2] ?

Thanks for your help.

Match is referring to the second line statement:
match = requestedURL.match(wikipediaDontRedirectRegExp);

This is using the match function to create the variable also named match. The returned value of the match function is an array, so the next two lines are grabbing the first two values out of this array and setting them to lang and page.

More details on the .match() function can be found here: https://www.w3schools.com/jsref/jsref_match.asp

In your case, you have the expression,

^https?://([a-zA-Z0-9\-_]+)\.(?:m\.)?wikipedia\.(?:com|org)/wiki/(.*)(?:\?oldformat=true|\?previous=yes)

The expression is looking for groups like ([a-zA-Z0-9-_]+) or (.*) that doesn' have a cancel, a quotion mark in front of it such as (?: blabla ).

Match is the results you get from the regular expression in your code. For each match, the elements you get in the array from element 1 to the size of the element are these groups.

For example, if we try to find the character couples that have space in between and 'u' after the couple, we use the code below.

Ex: The quick would return ["e qu", "e", "q"].

var paragraph = 'The quick brown fox jumps over the lazy dog. It barked.';
var regex = /(w) (w)u/;
var found = paragraph.match(regex);

console.log(found);
// Would return Array ["e qu", "e", "q"]

In Array ["e qu", "e", "q"]

The first element is the whole match, and the other elements found[1] and found[2] are the groups (the brackets) that I mentioned.

Alright, lemme see if I can do a breakdown on this regular expression for you. I'm far from an expert on using RegEx's but I'm sure there's plenty of people on this site who can point out my shortcomings with this.

Expression:
^https?://([a-zA-Z0-9\-_]+)\.(?:m\.)?wikipedia\.(?:com|org)/wiki/(.*)(?:\?oldformat=true|\?previous=yes)

^ character essentially having it check if it's a new line or start of input

http match should have this in it

s? match can optionally have "s" at this part

:// match should have this in it

( begin making a group to save for easy reference later

[a-zA-Z0-9\-_] match most letters, numbers, and special characters

+ previous defined group should contain one or more characters

) closing group marker

\. escape needed to include the . character in match

(?: create group but do not have this group saved for reference later

m\. match should have "m." in it

) closing group marker

? previous group made is optional

wikipedia\. match should have "wikipedia." in it

(?: create group but do not have this group saved for reference later

com|org match "com" or "org", they're interchangeable

) closing group marker

/wiki/ match should have "/wiki/" in it

( make a group that is saved for reference later

. match letters, numbers, or special characters

* include zero or more characters for previous marker

) closing group marker

(?: create group but do not save it

\? include "?" in match, meaning this is going to check queries put on the URL

\?oldformat=true|\?previous=yes check if "?oldformat=true" or "?previous=yes" queries are attached

) closing group marker

I think that about sums it up. If you have any more questions on it feel free to comment. But the gist of it is, match[1] is going to grab the ([a-zA-Z0-9\-_]+) group and the match[2] is going to grab the (.*) group after the /wiki/ directory portion.