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It looks like you're trying to replace <<:(anything here):>> .
str_replace might be a better option for you as it runs quicker - no need to run the regular expression engine.
If that won't work, try taking the call to process_element() out of quotes. It won't get run if it's in quotes. To be honest, I've never seen this done before, but I guess theoretically it should work.
Real programmers don't comment their code. If it was hard to write, it should be hard to understand.