It's actually pretty easy to detect multi-line inputs as long as they have clear enclosers (like '{}' for a block in C/C++). This is what reple does.

This doesn't require a parser--you just have to know what the enclosers are (one of the things that goes in a reple config file) and then count them to see if they're matched or not.

For languages without Python-like syntax or with enclosers that can be ambiguous (some dialects of Fortran), this would be pretty difficult.