Question on Pattern Matching

[Brandon Bay]

holly all,

I'm studying for my scjp 1.5, and pattern matching comes out unfortunately.

I do have some questions :

import java.util.regex.*;

class test{
public static void main(String[] args){
Pattern p = Pattern.compile("\\d");
Matcher m = p.matcher("ab34ef");
boolean b = false;
while(b = m.find()){
System.out.print(m.start() + m.group());
}
}
}

Is this true (My understanding)? : Find me groups of 1 of integer, and when you found one, print me out the index of the found integer based on the String and the group of 1 integer.

the output in compiler :

--------------------Configuration: <Default>--------------------
2334
Process completed.

However, if i add a * at the back of \\d, which is :

class test{
public static void main(String[] args){
Pattern p = Pattern.compile("\\d*");
Matcher m = p.matcher("ab34ef");
boolean b = false;
while(b = m.find()){
System.out.print(m.start() + m.group());
}
}
}

the compiler prints out :

--------------------Configuration: <Default>--------------------
01234456
Process completed.

Why is this so? I thought what i'm telling the compiler is "Hey compiler, find me groups which start with a digit, while you're on it, take anymore integers behind the first one and form that into a group. Then move on the next token."

I really hope someone can give me some feedback. My exam is like less than 2 weeks away, and i'm really new on this chapter. My lecturer said that we weren't taught this before, but he skipped it.. I mean, wth. lol

[Jesus Angeles]

Just to note that we are encouraged at Javaranch to use real sounding names. Not names of Singapore Idol winners, unless that is really your name.

[Brandon Bay]

Another question even I read the book, I still can't get it

heres what happened (Taken from Kathy/Berts' book) :

Let's see some of these formatting strings in action:

int i1 = -123;
int i2 = 12345;
System.out.printf(">%1$(7d< \n", i1);
System.out.printf(">%0,7d< \n", i2);
System.out.format(">%+-7d< \n", i2);
System.out.printf(">%2$b + %1$5d< \n", i1, false);
 
This produces:
 
> (123)<
>012,345<
>+12345 <
>false + -123<

I understand that % is needed, and 2$ represents the second element(or index) in the expression? What about the > ? Really hope to get someone to clarify




im just so lost

[Brandon Bay]

Originally posted by Jesus Angeles:
Just to note that we are encouraged at Javaranch to use real sounding names. Not names of Singapore Idol winners, unless that is really your name.

I'm really sorry, my name's Taufik actually, but did not really want to reveal my full name, so substituted half of it..

[Burkhard Hassel]

Howdy "Taufik M." !

Thanks for your first contribution to this forum and...

Welcome to the Ranch!




Hope you'll enjoy.




Only one small issue: The Java Ranch follows a certain policy regarding user names.
The main reasons why and a link how to change yours you'll find here:
http://www.javaranch.com/name.jsp


So, could you please change your user name before your next posting?
It will not affect anything you've already posted here. Just your user name will update.


I'm posting this because I am one of the moderators of this forum.


Yours,
Bu.

[marc weber]

Originally posted by Taufik M.:
...did not really want to reveal my full name.

That's okay, but please see the JavaRanch naming policy for how to handle that.

[Tony Smith]

If you are really concerned about revealing your real name, you can always put a fake name as long as it sounds like a real name.

I did some looking into your first question:

When you do Pattern.compile("\\d*"); it's saying find me a match with 0 or more digits. What you are really looking for is Pattern.compile("\\d+"); find me a match with 1 or more digits. If you replace * with a + that should do it.

As to why the * prints 01234456 is because when you have 0 or more digits, pretty much everything makes m.find() return true. At least that's what I think. So it goes by every charcter and return true:

a:true but empty group
b:true but empty group
3:true group:34
e:true but empty group
f:true but empty group

thus:0123445

as to the extra 6 in the end, it puzzles me a bit. Seems like there is a hidden character of some sort that is always attached to the end and always makes m.find() return true even though there is no group printed. When you use * or ?.

[Jesus Angeles]

Originally posted by Taufik M.:


I'm really sorry, my name's Taufik actually, but did not really want to reveal my full name, so substituted half of it..

I hope we did not scare you away. Just follow the name guidelines mentioned above. Someone already attempted to answer your question. I hope you stay with us in this friendly ranch.