<div>
Here's the actual code snippet. I only used the jsfiddle example because it demonstrates the code actually working. 
 

<pre><code class="code-10 nolinks" id="code-20-41495544-1">test1 = &quot;http://www.mywebsite.com&quot;
test2 = &quot;https://www.mywebsite.com&quot;
test3 = &quot;www.mywebsite.com&quot;

if (test1.startsWith(&quot;http://&quot;) || test1.startsWith(&quot;https://&quot;)) {
 alert('yes');
} else {
 alert('no');
}
if (test2.startsWith(&quot;http://&quot;) || test2.startsWith(&quot;https://&quot;)) {
 alert('yes');
} else {
 alert('no');
}
if (test3.startsWith(&quot;http://&quot;) || test3.startsWith(&quot;https://&quot;)) {
 alert('yes');
} else {
 alert('no');
}
</code></pre>
</div>

<div>
My code is wrong. &nbsp;url.substring(4) should be url.substring(0,4) 
 
<a href="http://jsbin.com/foxojuyiko/edit?html,output" rel="ugc">http://jsbin.com/foxojuyiko/edit?html,output</a> 

<pre><code class="code-10 nolinks" id="code-20-41495547-1">&lt;!DOCTYPE html&gt;
&lt;html&gt;
&lt;head&gt;
&lt;script src=&quot;https://code.jquery.com/jquery-2.1.4.js&quot;&gt;&lt;/script&gt;
 &lt;meta charset=&quot;utf-8&quot;&gt;
 &lt;meta name=&quot;viewport&quot; content=&quot;width=device-width&quot;&gt;
 &lt;script&gt;
 $(function(){
 var url = $('#inputURL').val();
 if(url.substring(0,4)!=='http'){
 url = 'http://'+url;
 }
 $('#result').html(url);
});
 &lt;/script&gt;
 &lt;title&gt;28931241&lt;/title&gt;
&lt;/head&gt;
&lt;body&gt;
&lt;input id=&quot;inputURL&quot; name=&quot;url&quot; value=&quot;google.com&quot;&gt;
 &lt;div id=&quot;result&quot;&gt;&lt;/div&gt;
&lt;/body&gt;
&lt;/html&gt;

 
</code></pre>
</div>

<div>
I would add that if you are posting this data to update a database that will eventually be used to display information requiring http or https, then you should also have the same test in your server side code and not rely 100% on client side code for this.
</div>

<div>
yes i want to preserve the protocol. it may start with ftp or anything else... 
is there any js library which can parse a url gracefully or one can easily do it with some simple code like Aviv
</div>

<div>
Hi ste5an's code looks good... but i came across a scenario where this wont work. 
eg. this is also a url : mailto:rohit@gmail.com 
But the above code wont detect it.... 
I found a library for parsing uri's : <a href="https://medialize.github.io/URI.js/build.html" rel="ugc">https://medialize.github.io/URI.js/build.html</a> 
this library detects the mailto: protocol too.. 
But since i just need to separate out the protocol part, i want to avoid using a library... and instead use some regular expression.... 
The code URI.js is minified.... dont know how exactly they are separating out the protocol part.
</div>

<div>
Yup. From <a href="http://www.rfc-editor.org/rfc/rfc3986.txt" rel="ugc">RFC3986</a>: 
 

<pre><code class="code-1 nolinks" id="code-20-41498707-1"> URI = scheme &quot;:&quot; hier-part [ &quot;?&quot; query ] [ &quot;#&quot; fragment ]
 scheme = ALPHA *( ALPHA / DIGIT / &quot;+&quot; / &quot;-&quot; / &quot;.&quot; )
</code></pre>
 
Thus you need to look for the existence of a colon and whether the part before is a valid scheme name.
</div>

<div>
<div class="content wysiwyg-content">
Hi, 
I have a user enterable link in my application. If the user does not specify the protocol then i want to add <a href="http://" rel="ugc nofollow">http://</a> by default to it. 
eg. if user enters <a href="http://www.google.com" rel="ugc">www.google.com</a> then i want to convert it to <a href="http://www.google.com" rel="ugc">http://www.google.com</a> 
else if user enters <a href="http://www.google.com" rel="ugc">http://www.google.com</a> or <a href="https://" rel="ugc nofollow">https://</a>... i want to keep it same. 
basically i want to modify only those strings and prepend it with <a href="http://" rel="ugc nofollow">http://</a>&nbsp;if no url is specified. 
 
How can i do it in javascript or jquery ? 
Thanks
</div>
</div>

Hi,
I have a user enterable link in my application. If the user does not specify the protocol then i want to add http:// by default to it.
eg. if user enters www.google.com then i want to convert it to http://www.google.com
else if user enters http://www.google.com or https://... i want to keep it same.
basically i want to modify only those strings and prepend it with http:// if no url is specified.

How can i do it in javascript or jquery ?
Thanks

Parsing url in javascript

JavaScript is a dynamic, object-based language commonly used for client-side scripting in web browsers. Recently, server side JavaScript frameworks have also emerged. JavaScript runs on nearly every operating system and  in almost every mainstream web browser.

JavaScript

jQuery (Core) is a cross-browser JavaScript library that provides abstractions for common client-side tasks such as Document Object Model (DOM) traversal, DOM manipulation, event handling, animation and Ajax. jQuery also provides a platform to create plugins that extend jQuery's capabilities beyond those already provided by the library.