Understanding the regex from a jQuery get url parameter function

Question

I have searched stack overflow for a Javascript solution (in the jQuery library) to get the URL parameter.

I have this function which does it smoothly:

// get the firstname value from the myurl - http://mysite.com?firstname=dany&lastname=dughy

var myurl = $("#gup").data("url");
var name = "firstname";

function gup(name, myurl) {
    name = name.replace(/[\[]/,"\\\[").replace(/[\]]/,"\\\]");
    var regexS = "[\\?&]"+name+"=([^&#]*)";
    var regex = new RegExp( regexS );
    var results = regex.exec(myurl);
    if( results == null )
        return "";
    else
        return results[1];
}

I understand how it works, you set a regex and than execute that regex and retrieve the results in an array.

Can someome please explain the regex to me? I can't seem to understand it.

Ty

Gerard Sexton · Accepted Answer · 2013-02-05 13:02:58Z

2

Let's call the function with gup("NAME","http://domain.com?NAME=value&name2=value2").

function gup(name, myurl) {
    name = name.replace(/[\[]/,"\\\[").replace(/[\]]/,"\\\]");

This line escapes square brakets so it works nicely in the regex query, making "[]" into "\\\[\\\]".

    var regexS = "[\\?&]"+name+"=([^&#]*)";

Now the name is parsed to work in the regex. It will look like "[\\?&]NAME=([^&#]*)".

"[\\?&]" means find either ? or &. Because ? is a special character, to match a ? in text you need to escape it like \?, so this regex would become [\?&] but in javascript strings you need to escape the \ again. So [\?&] becomes "[\\?&]" in a string.

=([^&#]*) matches a literal = followed by any character except an & or #.

    var regex = new RegExp( regexS );
    var results = regex.exec(myurl);
    if( results == null )
        return "";
    else
        return results[1];

Returns the captured group, which is at position 1. Position 0 is the whole match.

Function call returns value

answered Feb 5, 2013 at 13:02

Gerard Sexton

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5 Comments

Dany Dughila Over a year ago

I really did not know you had to escape strings again in JS (which confused me) and I definitely need to work some more on practical regexes. You opened a great path for me. Ty!

eithed Over a year ago

Won't [\\?&] find \, ? or & though? Regexp tester gives me this: i.imgur.com/UM0Pcok.png

Gerard Sexton Over a year ago

@eithed yes, it would if it were plain regex but it is in a JS string, so you need to escape the \ that is used for escaping the ?

Gerard Sexton Over a year ago

@eithed, you are testing it incorrectly. The point is being in a JS string. This shows what happens to a JS string: new RegExp("[\\?&]"). Then execute this: "i\\love?programming&too".replace(/[\?&]/gi,""). Besides, the \ is not used as a separator in URLs so the point is mute.

eithed Over a year ago

@Gerard Sexton, ah, yes, now I know what you mean. Thank you for explanation and your time!

eithed · Accepted Answer · 2013-02-05 12:36:44Z

0

Let's say that you're searching for firstname, then the regexp becomes: [\\?&]firstname=([^&#]*). Now, what it means is: [\\?&] search for any of characters \, ? or & that has firstname= afterwards, and [^&#]* = anything but & or # characters afterwards

answered Feb 5, 2013 at 12:36

eithed

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2 Comments

Dany Dughila Over a year ago

Ty very much for answering. I believe Gerard's answer is more comprehensive; this is why I have accepted it.

eithed Over a year ago

No worries :) Regexpes are funny like that. I think though that there's a small mistake in the explanation of [\\?&], as it means \, ? or & (well, at least I think it does :D). Let me put this under his answer as a comment.

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