I think it is good that you have checked to see whether the link provided by users in the Add Command is from NUSMods, however I notice that some mods could be added to the Person even though the mod does not exist
Steps to reproduce:
- Open nusmates.jar
- Type in "add n/tea l/https://nusmods.com/timetable/sem-1/share?CS1234=TUT:05,LAB:E08" to the input bar
Result: Person "tea" is added to NUSMates, with the mod CS1234 tagged to it (attached in screenshot)

I noticed that CS1234 is not an existing mod in NUSMods, maybe you could include more checks to see if the input mods exist, with possibly an error message to let users know that the mod does not exist / is not available
I think it is good that you have checked to see whether the link provided by users in the Add Command is from NUSMods, however I notice that some mods could be added to the Person even though the mod does not exist
Steps to reproduce:
Result: Person "tea" is added to NUSMates, with the mod CS1234 tagged to it (attached in screenshot)

I noticed that CS1234 is not an existing mod in NUSMods, maybe you could include more checks to see if the input mods exist, with possibly an error message to let users know that the mod does not exist / is not available