Difference of || and or
In short, they have different priorities.
The priority of or is much lower, even lower than that of =.
Forgetting this can easily cause problems.
from = "some random string"
to = "AAA"
from[/random/] = to or "DEFAULT"
# actually means
(from[/random/] = to) or "DEFAULT"
# and wlil never replace matched part with "DEFAULT"
# one should use
from[/random/] = to || "DEFAULT"Reference: this SO answer