Regex match on script tags that DO NOT contain certain attributes

Question

I'm battling building a regular expression for a html scan. The regex should match any <script> html element that does not contain either the async or the defer attributes.

Unit Test:

• <script defer src="a.js"> should not match
• <script src="a.js" defer> should not match
• <script src="a.js"> should match
• <script async src="a.js"> should not match
• <script src="a.js" async> should not match

I've fiddled with a solution for setting this up on single attributes (^.*<script((?!defer).)*$ and ^.*<script((?!async).)*$ respectively) and I also created one that matches if the html element does contain either attribute (<script.+(?=defer|async).+>) but I can't grok it the other way around.

Any ideas?

Answer 1

Your experiments were very close!

Try the following regular expression, and demonstrated with your examples here plus a few other use cases that might occur like <script test defer src="a.js"> or <script test src>.

<script(?:(?!defer|async).)*?>

Explanation

• <script - < character followed by the word script
• (?:(?!defer|async).)*? - negative lookahead for the word defer or async followed by a single character, matching multiple times if possible but preferring fewer matches (i.e. match a single character not preceded by the word defer or async, matching multiple times if possible but preferring fewer matches)
• > - > character

Tests

|--------------------------------|-----------|
| Use case                       | Matches   |
|--------------------------------|-----------|
| <script defer src="a.js">      | NO        |
| <script async src="a.js">      | NO        | 
| <script src="a.js">            | YES       |
| <script test src="a.js">       | YES       |
| <script test async src="a.js"> | NO        |
| <script src="a.js" defer>      | NO        |
| <script src="a.js" async>      | NO        |
| <script test src>              | YES       |
|--------------------------------|-----------|