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Reg Ex Question

 
Greenhorn
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Can some one clear below output , i am preety much confused ( From KB book)

import java.util.regex.*;
class Regex {
public static void main(String [] args) {
Pattern p = Pattern.compile(args[0]);
Matcher m = p.matcher(args[1]);
boolean b = false;
System.out.println("Pattern is " + m.pattern());
while(b = m.find()) {
System.out.println(m.start() + " " + m.group());
}
}
}

% java Regex "\d\w" "ab4 56_7ab"

Produces the output
Pattern is \d\w
4 56
7 7a


Please explain how this output is produced.

Ashish
 
Rancher
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76
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How about if you post how you think this works (or how it fails to work according to your understanding)? Explaining something is a great way to make sure you really understand it. It's also the approach we prefer here at JavaRanch.
 
Greenhorn
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Hi Ashish,
you have entered Exp "\d\w" "ab4 56_7ab"

The start() method returns the index of the matching pattern in your input string & group() method groups the matching pattern starting from the index position returned by start() method.

\d -search for a digit in the pattern
\w search for a word in the pattern,a word can be digit,letter or "_"

your output is:

4 56 (your first matching statrs from index 4,that is 5 and second match is 6(digit,letter,"_"))
7 7a (second match starts from index 7,that is 7 and second match is a(digit,letter,"_"))

you might be confuse with the underscore,here underscore is not in the result because the 6 just before underscore is already matched & is not included in the next matching.If you insert a new digit before underscore then you will surely get underscore in the result.

I hope i have provided your answer,if there is any confusion then reply me

Mamta
 
Ashish Soni
Greenhorn
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Thanks i got it now.
 
Ranch Hand
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Hi Mamta,

As you have mentioned that there is a digit in the pattern , the digit can start from the index 2 instead of 4 .As I see that the digit is starting from 2 the output should start from index no 2.

Please correct me if I am wrong.

thanks,
Jyothsna
 
Jyothsna Panchagnula
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Hello All,

Can any one throw some focus on the output from the program posted above?

Jyothsna
 
Ranch Hand
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Originally posted by Jyothsana:
As I see that the digit is starting from 2 the output should start from index no 2.



search string : "ab4 56_7ab"
pattern : "\d\w"
The pattern implies consecutive occurrence of a (digit)+(digit,letter,_)

Digit starts at index no.2 but the following character is a *space*, hence it does not match the pattern.
Moreover "6_" also matchs the pattern, but since "6" has already been used in the match "56" hence "6" does not qualify to give a match of "6_".
 
Don't get me started about those stupid light bulbs.
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