simple Matcher question

[Marc E]

All, given this code:

Pattern p = Pattern.compile("\\d*");
Matcher m = p.matcher("a");
while(m.find()){
System.out.println("start is " + m.start() );
}

Why is the output
0
1

?

I understand that start returns the index of the start of the search, but
given that this string is only one character, I don't understand how it gets
to an index of 1. does the start value of 1 indicate that it's actually
searching after the "a"?, i.e. that somehow find() is returning true after
it hits the end of the string?

Thanks for a clear explanation

 

[Jim Korman]

All, given this code:

Pattern p = Pattern.compile("\\d*");
Matcher m = p.matcher("a");
while(m.find()){
System.out.println("start is " + m.start() );
}

Why is the output
0
1

?

I understand that start returns the index of the start of the search, but
given that this string is only one character, I don't understand how it gets
to an index of 1. does the start value of 1 indicate that it's actually
searching after the "a"?, i.e. that somehow find() is returning true after
it hits the end of the string?

Thanks for a clear explanation

You're searching for <zero or more> occurances of a digit.

Matching "a" at position 0

then {I'm assuming} matching nothing at the end of the string.

Then no more data and exit.

Jim

 

[Chris Smith]

All, given this code:

Pattern p = Pattern.compile("\\d*");
Matcher m = p.matcher("a");
while(m.find()){
System.out.println("start is " + m.start() );
}

Why is the output
0
1

?

There are zero or more digits before the 'a', and there are zero or more
digits after the 'a'. If you had more characters, it would match
between each character, as well. When a regular expression matches an
empty substring at the end of a string, it returns the length of the
string. There's really no other option.

Most of the time, it's a mistake to use Matcher.find with a regular
expression that generates the empty string.

 

[Marc E]

cool. easy enough to digest. thanks guys.

There are zero or more digits before the 'a', and there are zero or more
digits after the 'a'. If you had more characters, it would match
between each character, as well. When a regular expression matches an
empty substring at the end of a string, it returns the length of the
string. There's really no other option.

Most of the time, it's a mistake to use Matcher.find with a regular
expression that generates the empty string.