Singular and double CONTAINSSTRING terms

I have a dataset with strength descriptors in a column which resembles the following;

Row 1: extremely weak

Row 2: very weak

Row 3: very weak to weak

Row 4: weak

I think I'm relatively happy how to use CONTAINSSTRING() as a custom column in order to identify double word strings like 'very weak' (which should return TRUE for Row 2 and Row 3 for example).

I've also been trying to think of a way to use CONTAINSSTRING() to return TRUE for just the term 'weak' (i.e. False (Row 1), False (Row 2), True (Row 3), True (Row 4)) but I'm struggling to think of the best way to achieve this? I was thinking of filtering out 'very weak' first, but that would mean filtering out Row 3 which also contains the singular 'weak' term. 

There might be a simple solution, but my brain is struggling with the logic.