How to extract a variable sub-string from a string [duplicate]


This question already has an answer here:

<ul><li> <a href="/questions/901115/how-can-i-get-query-string-values-in-javascript" dir="ltr" rel="nofollow">How can I get query string values in JavaScript?</a> <span class="question-originals-answer-count"> 73 answers </span> </li> </ul>

I want to extract a sub-string from a string using javascript, then replace as set of characters by another character.

I know the prefix and postfix of the string. Between the prefix and postfix is the variable sub-string that I want to extract and replace set of characters on it.

For example, this string represents error URL. I need to extract the URL from it which in this example is: https%3A//revoked.badssl.com/ then replace %3A with :

The logic is that, the prefix is: about:neterror?e=nssFailure2&u= and the postfix is &c=UTF* as I do not care about the rest of the string after the & character.

I probably need to use Regex. However, I know how to use Regex to compare whether a string matches a specific pattern or not. But I wonder how to use Regex to extract a sub-string?

about:neterror?e=nssFailure2&u=https%3A//revoked.badssl.com/&c=UTF8& f=regular d=An%20error%20occurred%20during%20a%20connection%20to%20revoked.badssl.com. %0A%0APeer%E2%80%99s%20Certificate%20has%20been%20revoked.%0A%0AError%20code %3A%20%3Ca%20id%3D%22errorCode%22%20title%3D%22SEC_ERROR_REVOKED_CERTIFICATE %22%3ESEC_ERROR_REVOKED_CERTIFICATE%3C/a%3E%0A

EDIT: When I try this script:

var myString="about:neterror?e=nssFailure2&u=https%3A//abcde.somethig/&c=UTF-8&f=regular&d=An%20error%20occurred%20during%20a%20connection%20to%20abcde.somethig.\ %0A%0ACannot%20communicate%20securely%20with%20peer%3A%20no%20common%20encryption%20algorithm%28s%29.%0A%0AError%20code%3A%20%3Ca%20id%3D%22errorCode%22%20\ title%3D%22SEC_ERROR_REVOKED_CERTIFICATE%22%3ESEC_ERROR_REVOKED_CERTIFICATE%3C/a%3E%0A"; console.log(myString.replace(/^about:neterror\?e=nssFailure2&u=(https)%3A(.*)&c=UTF8.*$/, "$1:$2"));

NOTE: I used \ at the end of the line just to break the string in the editor. But it is not part of the original string. I get output identical to the input.

<strong>NOTE2:</strong> What about if I want to just use & as mark of strat for the postfix? I used console.log(myString.replace(/^about:neterror\?e=nssFailure2&u=(https)%3A(.*)&.*$/, "$1:$2")); but it prints: https://abcde.somethig/&c=UTF-8&f=regular and I want: https://abcde.somethig/


Simplest I can think of without using some complex regex and assuming the &c and &u are static, is this - first decoding the string as suggested by Jedi

<pre class="snippet-code-js lang-js prettyprint-override">var str = "about:neterror?e=nssFailure2&u=https%3A//revoked.badssl.com/&c=UTF8&f=regulard=An%20error%20occurred%20during%20a%20connection%20to%20revoked.badssl.com.%0A%0APeer%E2%80%99s%20Certificate%20has%20been%20revoked.%0A%0AError%20code%3A%20%3Ca%20id%3D%22errorCode%22%20title%3D%22SEC_ERROR_REVOKED_CERTIFICATE%22%3ESEC_ERROR_REVOKED_CERTIFICATE%3C/a%3E%0A" var url = decodeURIComponent(str).split("&u=")[1].split("&c")[0]; console.log(url)

And here is why I close as duplicate:

<pre class="snippet-code-js lang-js prettyprint-override">function getParameterByName(name, url) { if (!url) url = window.location.href; name = name.replace(/[\[\]]/g, "\\$&"); var regex = new RegExp("[?&]" + name + "(=([^&#]*)|&|#|$)"), results = regex.exec(url); if (!results) return null; if (!results[2]) return ''; return decodeURIComponent(results[2].replace(/\+/g, " ")); } var str = "about:neterror?e=nssFailure2&u=https%3A//revoked.badssl.com/&c=UTF8&f=regulard=An%20error%20occurred%20during%20a%20connection%20to%20revoked.badssl.com.%0A%0APeer%E2%80%99s%20Certificate%20has%20been%20revoked.%0A%0AError%20code%3A%20%3Ca%20id%3D%22errorCode%22%20title%3D%22SEC_ERROR_REVOKED_CERTIFICATE%22%3ESEC_ERROR_REVOKED_CERTIFICATE%3C/a%3E%0A" var url = getParameterByName("u",str); console.log(url);


What you are trying to do is to retrieve a query parameter from a URL, and then decode it. For this, you should use libraries build for the purpose, not regexp. For example, many regexp approaches will depend on the precise order of the query params, which is something you cannot really (or should not) rely on.

Here's an example using the <a href="https://developer.mozilla.org/en/docs/Web/API/URL" rel="nofollow">URL API</a>, which is stable and <a href="http://caniuse.com/#feat=url" rel="nofollow">widely supported</a> (other than IE11, and a <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=1377772" rel="nofollow">bug in Firefox</a> for non-http-type URLs such as about:neterror currently in the process of being fixed):

<pre class="snippet-code-js lang-js prettyprint-override">var str = "about:neterror?e=nssFailure2&u=https%3A//revoked.badssl.com/&c=UTF8&f=regulard=An%20error%20occurred%20during%20a%20connection%20to%20revoked.badssl.com.%0A%0APeer%E2%80%99s%20Certificate%20has%20been%20revoked.%0A%0AError%20code%3A%20%3Ca%20id%3D%22errorCode%22%20title%3D%22SEC_ERROR_REVOKED_CERTIFICATE%22%3ESEC_ERROR_REVOKED_CERTIFICATE%3C/a%3E%0A"; var url = new URL(str); console.log(url.searchParams.get('u'));


You can capture the sub string you need and then use back reference to extract and reformat it:

<pre class="snippet-code-js lang-js prettyprint-override">var s = "about:neterror?e=nssFailure2&u=https%3A//revoked.badssl.com/&c=UTF8&f=regular d=An%20error%20occurred%20during%20a%20connection%20to%20revoked.badssl.com.%0A%0APeer%E2%80%99s%20Certificate%20has%20been%20revoked.%0A%0AError%20code%3A%20%3Ca%20id%3D%22errorCode%22%20title%3D%22SEC_ERROR_REVOKED_CERTIFICATE%22%3ESEC_ERROR_REVOKED_CERTIFICATE%3C/a%3E%0A" var url = s.replace(/^about:neterror\?e=nssFailure2&u=(https)%3A(.*)&c=UTF[\s\S]*$/, "$1:$2") console.log(url)

Using decodeURIComponent:

<pre class="snippet-code-js lang-js prettyprint-override">var s = "about:neterror?e=nssFailure2&u=https%3A//revoked.badssl.com/&c=UTF8&f=regular d=An%20error%20occurred%20during%20a%20connection%20to%20revoked.badssl.com.%0A%0APeer%E2%80%99s%20Certificate%20has%20been%20revoked.%0A%0AError%20code%3A%20%3Ca%20id%3D%22errorCode%22%20title%3D%22SEC_ERROR_REVOKED_CERTIFICATE%22%3ESEC_ERROR_REVOKED_CERTIFICATE%3C/a%3E%0A" var url = decodeURIComponent(s).replace(/^about:neterror\?e=nssFailure2&u=(.*)&c=UTF[\s\S]*$/, "$1") console.log(url)


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