Formal:
* സ്വാഗതം തിരിച്ചെത്താൻ (Swagatam thirichchethan) - This translates directly to "Welcome back," and is a very polite and formal way to greet someone.
* തിരിച്ചെത്തിയതിന് സ്വാഗതം (Thirichchethiyaathinu swagatam) - This means "Welcome for coming back," and is also quite formal.
Informal:
* തിരിച്ചെത്തിയതിന് നന്ദി (Thirichchethiyaathinu nandi) - This means "Thank you for coming back," and is a more casual and friendly way to greet someone.
* മടങ്ങി വന്നതിന് സ്വാഗതം (Madangi vannathinu swagatam) - This translates to "Welcome for returning," and is also informal.
You can also use the phrase "നല്ലത്, തിരിച്ചെത്തി (Nallath, thirichchethi)" which means "Good, you're back" for a casual and friendly welcome.
Ultimately, the best way to say "welcome back" in Malayalam depends on the specific situation and your relationship with the person you are greeting.