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Pattern Matching
First look at the normal code with +
greedy quantifier. In this example we try to find sequences containing at least one
a
character. Hence our pattern is a+
Pattern p = Pattern.compile("a+");
Matcher m = p.matcher("ab4 5aaaa6_7ab");
System.out.println("Pattern is " + m.pattern());
while(m.find()) {
System.out.println(m.start() + " " + m.group());
}
We'll get quite expectable output
Pattern is a+
0 a
5 aaaa
12 a
+
sign is a greedy quantifier, therefore we got all a
symbols starting from 5th position in one group. Now let's change
+
to *
greedy quantifier in our pattern and execute the same code with a*
pattern.
0 a
1
2
3
4
5 aaaa
9
10
11
12 a
13
14
As you see, we got a strange result. Do you have any idea about this result?
I'll try to explain. Because of the nature of *
quantifier, matcher looks for zero or more
occurrences of the a
character. That's why it gives every position in result, even after last character.
We'll have the similar result with ?
quantifier. Only the line 5 will be spanned to 4 lines with a single a
in each line.
Author: Jafar N.Aliyev (Jsoft)
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