Question

Why does this code throw a InputMismatchException ?

Scanner scanner = new Scanner("hello world");
System.out.println(scanner.next("hello\\s*world"));

The same regex matches in http://regexpal.com/ (with \s instead of \\s)

Answer 1

A Scanner, as opposed to a Matcher, has built in tokenization of the string, the default delimiter is white space. So your “hello world” is getting tokenized into “hello” “world” before the match runs. It would be a match if you changed the delimiter before scanning to something not in the string, eg.:

Scanner scanner = new Scanner("hello world");
scanner.useDelimiter(":");
System.out.println(scanner.next("hello\\s*world"));

but it seems like really for your case you should just be using a Matcher.

This is an example of using a Scanner “as intended”:

   Scanner scanner = new Scanner("hello,world,goodnight,moon");
   scanner.useDelimiter(",");
   while (scanner.hasNext()) {
     System.out.println(scanner.next("\\w*"));
   }

output would be

hello
world
goodnight
moon