What is the difference between
scanf(" %[^\n]", str_1);
and
scanf("%[^/n],str_2);
I am trying to read a full line as an input using scanf function, the first one works fine but as soon as I remove the space between " and % the function takes no input.
scanf(" %[^\n]", str_1);
This will skip whitespace (until it finds a non-whitespace character) and then start reading characters into str_1 until it reaches a newline or EOF. It will not do any bounds checking, so may overflow the storage for str_1. If an EOF is read before finding a non-whitespace character, it will return 0 without writing anything into str_1 (not even a NUL character).
scanf("%[^/n],str_2);
This will read characters other than / and n into str_2 until it sees either a / or n or an EOF. Like the first one, it does not do any bounds checking so may (will likely?) overflow the storage of str_2. If the first character of input is either a / or a n (or EOF) it will fail to match the pattern at all, returning 0 and not storing anything into str_2 (not even a NUL terminator).
Basic scanf semantics -- whitspace in the format string will skip 0 or more whitespace characters in the input until a non-whitespace character is reached to start whatever the next pattern is. %[ matches characters with no bounds checking, so should never be used with untrusted input -- you should always use an explicit bound or m modifier.
If you want to read lines of input (as opposed to whitespace delimited words, ignoring differences between newlines and other whitespace), you should use fgets or getline
