Replace Text with hyperlinks

This is a fairly complex PHP (rather than WP) issue if you want to take into account all possibilities.

If you replace test with <a href="#">test</a> you will run into problems if the content already has <a href="https://wordpress.stackexchange.com/questions/317374/?">test</a>, because the result will then be <a href="https://wordpress.stackexchange.com/questions/317374/?"><a href="#">test</a></a>

So, before you do the replace you will need to see if test is already a link. This is not that simple, because you cannot simply scan for <a href="https://wordpress.stackexchange.com/questions/317374/...">test</a>. After all, you may have something like <a href="https://wordpress.stackexchange.com/questions/317374/...">this is my test, yeah!. Or test may be a substring of another word, like protesting. To prevent the latter being targeted you might check for test, but then you may miss test,.

Another thing to consider: if there already is a link around test, do you want to keep that one or replace it?

So, depending on you needs you will have to build a regular expression that does complex pattern recognition on your content to make sure it does exactly what you need. For help with regular expressions, try Stack Overflow.

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