Please explain the working of * greedy quantifier

Question

Pattern ptn  = Pattern.compile(\"a*\");
Matcher mtch  = ptn.matcher(\"bbaac\");
if(mtch.find()){
    System.out.println(mtch.group());
}

Output - print

Answer 1

The issue is that there are 5 matches, and you only check for the first one which is an empty string as a* can match an empty string (to be more exact, it matches the empty space before the character it cannot match and at the end of string).

Use while instead of if.

See IDEONE demo:

String s = "bbaac";
Pattern pattern = Pattern.compile("a*");
Matcher matcher = pattern.matcher(s);
while (matcher.find()){
    System.out.println(matcher.group(0)); 
} 

The number of matches is 5:

• empty string before the first b
• empty string before the second b
• aa
• empty string before the c and
• empty (end of) string after c.

Using a+, with the + quantifier meaning 1 or more occurrences, no empty matches will get extracted and you will only get aa.

See the empty strings on regex101.com: