Replace QueryString value using regular expressions

Question

In C#, I'm trying to use regular expressions to replace values in a querystring. So, if I have:

http://www.url.com/page.aspx?id=1

I'd like to write a function where I pass in the url, the querystring value and the value to replace. Something along the lines of:

string url = "http://www.url.com/page.aspx?id=1";
string newURL = ReplaceQueryStringValue(url, "id", "2");

private string ReplaceQueryStringValue(string url, string replaceWhat, string replaceWith)
{
    return Regex.Replace(url, "[^?]+(?:\?"+replaceWhat+"=([^&]+).*)?",replaceWith);
}

Alex Baranosky · Accepted Answer · 2009-09-14 20:59:35Z

11

Here is a function that would do the job:

static string replace(string url, string key, string value)
{
    return Regex.Replace(
        url, 
        @"([?&]" + key + ")=[^?&]+", 
        "$1=" + value);
}

edited Sep 14, 2009 at 20:59

Alex Baranosky

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answered Sep 14, 2009 at 20:16

Andrew Hare

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5 Comments

David Andres Over a year ago

@Andrew: Looks like you're replacing keys, not values. Please adjust.

brism Over a year ago

@David: are you sure? It looks right to me. The parentheses around the possible ? or & and the key in the regex allows Andrew to include the original key in the replacement string using the backreference $1. The old value is discarded by only including the new value in the replacement string.

Andrew Hare Over a year ago

@brism - Thanks, that is exactly what is going on. @GordonG - I am curious about the edit, are you aware that string and String are synonymous in C#?

David Andres Over a year ago

@brism, Andrew: I may have been off my rocker earlier today. What I currently see in this answer is correct.

krystan honour Over a year ago

if I get a blank value in my query string so &foo= is this going to fail to match ?

Community · Accepted Answer · 2017-05-23 11:48:40Z

2

It might be easier to use String.Split to initially cut the URL into page and query string parts, and then use Split.String again to cut the query string into distinct parts.

var urlSplit = url.Split('?');
var originalURL = urlSplit[0];

var urlRedefined = url;   
if(urlSplit.Length == 2)
{
  var queryString = urlSplit[1].Split('&');

  //your code here

  var urlRedefined = String.Format("{0}?{1}", 
    originalURL, 
    String.Join("&", queryString);
}

A regular expression may be overkill for your needs. Also, the System.Uri class might be a better fit. Its use is covered in URL split in C#?.

edited May 23, 2017 at 11:48

CommunityBot

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answered Sep 14, 2009 at 20:10

David Andres

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2 Comments

Kris B Over a year ago

I probably could use String.Split. Why would regex be overkill?

David Andres Over a year ago

@unknown: I mean overkill in that regex patterns are notoriously painful to get right. You might see success in limited testing, but then some small thing can throw your code off. My answer doesn't give you an easy way to take a querystring's name (e.g,. "Id") and replace its value, so perhaps another approach will be more successful.

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