What exactly does this format string do when paired with fscanf?

Question

I am looking at some code and came across this line:

fscanf(file, "%*[: ]%16s", dest);

What does the %*[: ]%16s format string specifier do?

Answer 1

This format string

"%*[: ]%16s"

means that all symbols ':' and ' ' (the symbols placed in the square brackets in the format string) must be skipped in the input stream and then at most 16 characters be read in a character array.

In the format string the symbol * is assignment-suppressing character.

Here is a demonstration program. For visibility I am using sscanf instead of fscanf.

#include <stdio.h>

int main( void ) 
{
    const char *stream = "::: : : : :::Hello";
    char s[17];
    
    sscanf( stream, "%*[: ]%16s", s );
    
    printf( "\"%s\"\n", s );

    return 0;
}

The program output is

"Hello"