• 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