Use scanf with Regular Expressions

问题

I've been trying to use regular expressions on scanf, in order to read a string of maximum n characters and discard anything else until the New Line Character. Any spaces should be treated as regular characters, thus included in the string to be read. I've studied a Wikipedia article about Regular Expressions, yet I can't get scanf to work properly. Here is some code I've tried:

scanf("[ ]*%ns[ ]*[\n]", string);

[ ] is supposed to go for the actual space character, * is supposed to mean one or more, n is the number of characters to read and string is a pointer allocated with malloc. I have tried several different combinations; however I tend to get only the first word of a sentence read (stops at space character). Furthermore, * seems to discard a character instead of meaning "zero or more"...

Could anybody explain in detail how regular expressions are interpreted by scanf? What is more, is it efficient to use getc repetitively instead?

Thanks in Advance :D

回答1:

The short answer: scanf does not handle regular expressions literally speaking.

If you want to use regular expressions in C, you could use the regex POSIX library. See the following question for a basic example on this library usage : Regular expressions in C: examples?

Now if you want to do it the scanf way you could try something like

scanf("%*[ ]%ns%*[ ]\n",str);

Replace the n in %ns by the maximal number of characters to read from input stream. The %*[ ] part asks to ignore any spaces. You could replace the * by a specific number to ignore a precise number of characters. You could add other characters between braces to ignore more than just spaces.

Not sure if the above scanf would work as spaces are also matched with the %s directive.
I would definitely go with a fgets call, then triming the surrounding whitespaces with something like the following: How do I trim leading/trailing whitespace in a standard way?