Regex for parsing parameters from url

Question

I'm a total noob with regexes and although I was trying hard I cannot create proper regexes to perform the following operation :

take url and check if it has a '?' followed by number with varying amount of digits.
if the match is correct, get the number after the '?' sign
exchange this number with different one.

So let's say we have this url :

http://website.com/avatars/avatar.png?56

we take '56' and change it to '57'.

I have the following regex for searching, I'm not sure if it's proper :

\?[0-9]+

But I have no idea how to take ? away. Should I just throw it away from the string and forget about using regex here ? Then the replace part is the only one left.

Is there a specific reason to use a regex? You could use the location object, respectively location.search. — Jürgen Thelen
– Jürgen Thelen, Commented Jul 8, 2011 at 15:00
Your regex is a-ok. You just need to add back in the ? you take out. See my answer below. — ridgerunner
– ridgerunner, Commented Jul 8, 2011 at 16:44

The Mask · Accepted Answer · 2011-07-08 14:57:46Z

3

Try this:

var url = "http://website.com/avatars/avatar.png?56"; 
var match = url.match(/\?(\d+)/); 
if(match != null) {
   url = url.replace(match[1], "new number");
}

answered Jul 8, 2011 at 14:57

The Mask

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1 Comment

ridgerunner Over a year ago

Actually, no, matches[1] will be "56", and the replace statement is effectively: url = url.replace('56', "new number"); and the URL value ends up being: website.com/pathnew number/avatar.png?56.

ridgerunner · Accepted Answer · 2011-07-08 15:42:55Z

1

Your original regex will work just fine, just add back in the ? you are taking out like so:

var newnum = 57;
url = url.replace(/\?[0-9]+/, '?'+ newnum);

answered Jul 8, 2011 at 15:42

ridgerunner

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Comments

ianbarker · Accepted Answer · 2011-07-08 14:55:59Z

0

I'm no regex expert but I think you can use a lookaround to ignore the '?'

(?<=?)([0-9]+)

which should give you your number in the first match

answered Jul 8, 2011 at 14:55

ianbarker

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2 Comments

The Mask Over a year ago

this is very simple,you can split the matched values using the () there an example in above my post...

ridgerunner Over a year ago

Javascript does not have lookbehind.

Brad Christie · Accepted Answer · 2011-07-08 14:59:04Z

0

VERY dummied-down approach:

$('#parse').click(function(e){
    var fromUrl = $('#from-url').val();
    var newNum = parseInt($('#new-number').val(), 10);

    var urlRE = /(?!\?)(\d+)$/;
    if (urlRE.test(fromUrl)){
        $('#result').text(fromUrl.replace(urlRE, newNum));
    }else{
        $('#result').text('Invalid URL');
    }
});

DEMO

There are not extravagant check-sums, error-checking, etc. Fromt here, use window.location or a string containing the URL if necessary.

Broken out in to a function (demo):

// Call this to replace the last digits with a new number within a url.
function replaceNumber(url, newNumber){
    // regex to find (and replace) the numbers at the end.
    var urlRE = /\?\d+$/;

    // make sure the url end in a question mark (?) and
    // any number of digits
    if (urlRE.test(url)){
        // replace the ?<number> with ?<newNumber>
        return url.replace(urlRE, '?'+newNumber);
    }

    // invalid URL (per regex) just return same result
    return url;
}

alert(replaceNumber('http://website.com/avatars/avatar.png?56', 57));

edited Jul 8, 2011 at 14:59

answered Jul 8, 2011 at 14:53

Brad Christie

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3 Comments

ridgerunner Over a year ago

The expression /(?!\?)(\d+)$/ is equivalent to /(\d+)$/ (The negative lookahead assertion will always be true at the position of a digit!)

Brad Christie Over a year ago

@ridgerunner: They wanted to confirm that the link ended with a question mark and numbers. The negative look-ahead validates that. (Unless I'm misunderstanding what you're stating)

ridgerunner Over a year ago

No, the negative lookahead does no such thing. It is a zero width assertion applied immediately preceding the first digit - it does not verify that the character before the first digit is a question mark. Remember that lookahead and lookbehind assertions are applied between characters, In this case, between the question mark (if its actually there), and the first digit.

Jason Gennaro · Accepted Answer · 2011-07-08 15:01:48Z

0

You could do this without regex.

var newNum = "57";
var url = "http://website.com/avatars/avatar.png?56";
var sUrl = url.split('?');
var rUrl = sUrl[0] + "?" + newNum;
alert(rUrl);

Split the URL at the ?
This returns an array.
Add the first item in the array and the ? and the new number back together.

http://jsfiddle.net/jasongennaro/7dMur/

answered Jul 8, 2011 at 15:01

Jason Gennaro

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2 Comments

ridgerunner Over a year ago

What if there is more than one ? character? (The ? is valid in both ?query and #fragment.) Also, what if there are more query parameters (or a #fragment) following the number?

Jason Gennaro Over a year ago

Thx @ridgerunner. I didn't consider any of those scenarios because the OP said check if it has a ? followed by number with varying amount of digits and his example was http://website.com/avatars/avatar.png?56 I figured it was pretty clear cut. But agreed, a more complex URL would require something else.

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