usage of % [^\n]

Question

A[50][5000];
for(i=0;i<50;++i)
scanf("%[\n]",A[i]);

%[^\n]  
 usage and meaning of it

and can i use that struct like

       %[\t]
       %[\a]

Answer 1

copies a string up to a newline from standard input to element i of A. as written, this acts almost like gets().

Answer 2

scanf()'s "%[" conversion specifier starts what's called a "scanset". It's has some similarities to the regex construct that looks the same (but it still is quite different) Here's what the standard says:

Matches a nonempty sequence of characters from a set of expected characters (the scanset).

...

The conversion specifier includes all subsequent characters in the format string, up to and including the matching right bracket (]). The characters between the brackets (the scanlist) compose the scanset, unless the character after the left bracket is a circumflex (^), in which case the scanset contains all characters that do not appear in the scanlist between the circumflex and the right bracket. If the conversion specifier begins with [] or [^], the right bracket character is in the scanlist and the next following right bracket character is the matching right bracket that ends the specification; otherwise the first following right bracket character is the one that ends the specification. If a - character is in the scanlist and is not the first, nor the second where the first character is a ^, nor the last character, the behavior is implementation-defined.

So the scanf() conversion "%[\n]" will match a newline character, while "%[^\n]" will match all characters up to a newline.

Here's what P.J. Plauger has to say about scansets in "The Standard C Library":

A scan set behaves much like the s conversion specifier. It stores up to w characters (default is the rest of the input) in the char array pointed at by ptr. It always stores a null character after any input. It does not skip leading white-space. It also lets you specify what characters to consider as part of the field. You can specify all the characters that match, as in %[0123456789abcdefABCDEF], which matches an arbitrary sequence of hexadecimal digits. Or you can specify all the characters that do not match, as in %[^0123456789] which matches any characters other than digits.

If you want to include the right bracket (]) in the set of characters you specify, write it immediately after the opening [ (or [^), as in %[][] which scans for square brackets. You cannot include the null character in the set of characters you specify. Some implementations may let you specify a range of characters by using a minus sign (-). The list of hexadecimal digits, for example, can be written as %[0-9abcdefABCDEF] or even, in some cases, as %[0-9a-fA-F]. Please note, however, that such usage is not universal. Avoid it in a program that you wish to keep maximally portable.

Answer 3

Yes, it's pretty much like a set in a regular expression -- you can specify a set of character to be accepted, or a set of characters to end the scan, so "%[^ \r\n\t]" would read until it encountered a space, carriage return, new-line or tab. Like with an RE, the leading "^" means "not" -- you can omit it to specify the characters that will be accepted instead of those that will end the conversion. With most compilers (though it's not technically required) you can specify ranges, such as "%[a-z]" to specify any lower-case letter (in this case, where the '-' isn't the first or last character, the behavior is implementation defined).

Though not widely used (or even known) this conversion has been part of C almost forever, and is supported in C89/90.