Question

In java i would like to read a file line by line and print the line to the output.
I want to solve this with regular expressions.

while (...)
{
  private static java.util.regex.Pattern line = java.util.regex.Pattern.compile(".*\\n");
  System.out.print(scanner.next(line));
}

The regex in the code is not correct, as i get InputMismatchException.
I am working on this regex for 2 hours. Please help with it.

With regex powertoy i see that “.*\n” is correct. But my program runs incorrectly.

The whole source is:

/**
 * Extracts the points in the standard input in off file format to the standard output in ascii points format.
 */

 import java.util.regex.Pattern;
 import java.util.Scanner;

class off_to_ascii_points 
{
    private static Scanner scanner = new Scanner(System.in);    
    private static Pattern fat_word_pattern = Pattern.compile("\\s*\\S*\\s*");
    private static Pattern line = Pattern.compile(".*\\n", Pattern.MULTILINE);

    public static void main(String[] args) 
    {
        try
        {
            scanner.useLocale(java.util.Locale.US);

                    /* skip to the number of points */
            scanner.skip(fat_word_pattern);

            int n_points = scanner.nextInt();

                    /* skip the rest of the 2. line */
            scanner.skip(fat_word_pattern); scanner.skip(fat_word_pattern);

            for (int i = 0; i < n_points; ++i)
            {
                    System.out.print(scanner.next(line));
                      /*
                      Here my mistake is. 
                      next() reads only until the delimiter, 
                      which is by default any white-space-sequence. 
                      That is next() does not read till the end of the line 
                      what i wanted.

                      Changing "next(line)" to "nextLine()" solves the problem.
                      Also, setting the delimiter to line_separator 
                      right before the loop solves the problem too.
                      */
            }

        }
        catch(java.lang.Exception e)
        {
            System.err.println("exception");
            e.printStackTrace();
        }
    }
}

The beginning of an example input is:

OFF
4999996 10000000 0
-28.6663 -11.3788 -58.8252 
-28.5917 -11.329 -58.8287 
-28.5103 -11.4786 -58.8651 
-28.8888 -11.7784 -58.9071 
-29.6105 -11.2297 -58.6101 
-29.1189 -11.429 -58.7828 
-29.4967 -11.7289 -58.787 
-29.1581 -11.8285 -58.8766 
-30.0735 -11.6798 -58.5941 
-29.9395 -11.2302 -58.4986 
-29.7318 -11.5794 -58.6753 
-29.0862 -11.1293 -58.7048 
-30.2359 -11.6801 -58.5331 
-30.2021 -11.3805 -58.4527 
-30.3594 -11.3808 -58.3798 

I first skip to the number 4999996 which is the number of lines containing point coordinates. These lines are that i am trying to write to the output.

Answer 1

I suggest using

private static Pattern line = Pattern.compile(".*");

scanner.useDelimiter("[\\r\\n]+"); // Insert right before the for-loop

System.out.println(scanner.next(line)); //Replace print with println

---

Why your code doesn’t work as expected:

This has to do with the Scanner class you use and how that class works.

The javadoc states:

A Scanner breaks its input into tokens
using a delimiter pattern, which by
default matches whitespace.

That means when you call one of the Scanner’s.next* methods the scanner reads the specified input until the next delimiter is encountered.

So your first call to scanner.next(line) starts reading the following line

-28.6663 -11.3788 -58.8252 

And stops at the space after -28.6663. Then it checks if the token (-28.6663) matches your provided pattern (.*\n) which obviously doesn’t match (-28.6663). That’s why.