A shorthand memory hook to remember "Tarzan"|(Tarzan) is that this is similar to conditional evaluation of boolean expressions. For example, in Python you often do

foo = foo or [ 23, 42 ]

Or more generally:

foo = foo or ConstructSomeFoo()

If foo is None then it gets this default value or the newly constructed object, otherwise it's unchanged. Key here is that what's after "or" is not even evaluated if the first operand is already evaluating to True.

So, the left "Tarzan" eats up the matching substring that we do not want, while the right (Tarzan) matches what we do want, but only if the left one didn't already hit.