Escape Sequence doubt

[saloni jhanwar]

It works fine with "\d" whereas with "\\d" why does it gives output like this ? while i have to use "c:\\scjp\\mytxt.txt" for IO operations.

class SplitMethod 
{
	public static void main(String[] args) 
	{
		if((args[0]!=null)&&(args[1]!=null)){
		String[] tokens=args[0].split(args[1]);
		for(String s:tokens)
			System.out.println(s);
		}
	}
}

Output : -

C:\SCJP\Chapter6\Tokenizing>java SplitMethod "as3ds65df6" "\d"
as
ds

df

C:\SCJP\Chapter6\Tokenizing>java SplitMethod "as3ds65df6" "\\d"
as3ds65df6

[Matthew Brown]

Are you wanting to split on the digits? In that case, the correct regular expression is \d. \\d means something else as a regular expression: it would look to split on a '\' character followed by a 'd'.

I'm guessing the source of your confusion is that you're thinking an extra \ is needed to escape the \. But that's when you're putting the regular expression into a Java string literal. When you're passing the regular expression in as a command-line argument that isn't necessary.

So with the code you've got, you call it with the line java SplitMethod "as3ds65df6" "\d". But using a string literal, the following code would give the same result:

class SplitMethod {  
    public static void main(String[] args) { 
        String[] tokens=args[0].split("\\d");  
        for(String s:tokens)  
            System.out.println(s);  
        }  
    }  
}  

[saloni jhanwar]

From book k&B 504 page

So, what if you need to search for periods (.) in your source data? If you just put a period in the regex expression, you get the "any character" behavior. So, what if you try \. ? Now the Java compiler thinks you're trying to create an escape sequence that doesn't exist. The correct syntax is

String s = "ab.cde.fg";
String[] tokens = s.split("\\.");

but if i do so then it doesn't work. for me it is correct "\." not "\\."for dot.

[saloni jhanwar]

thanks Matthew Brown, i got it.

[Helen Ma]

Hi, Saloni,
Maybe you can try this:

       String[] tokens = s.split("\\\\.");
   

to see if it works for a dot sign in a regex.

According to the book, \\. represents a dot. In the Java code, \\ represents a \. Therefore, \\\\ represents \\