$str = "
This is a test.  This test is > that test, but is less than < the other test.   The previous test may be  than the third test.   The fourth test may be < the first, but > than the last.  This is the end of the test string.
";

Need to identify and convert the legit, non HTML greater/less than signs to ASCII.. I'm running out of ideas, especially with the substrings:

The previous test may be < the second test, but is likely > than the third test.

and

The fourth test may be < the first, but > than the last.

Anyone have a suggestion or hint on how to accomplish this?