Regular Expression fails to matches() pattern with asterisk sign

Question

The sample code:

    String test = \"Z\";
    Pattern pt = Pattern.compile(\"[0-9]*\");
    Matcher mc = pt.matcher(test);
    System.out.println(mc.find());
            


        

Answer 1

The pattern [0-9]* says "matches 0 or more times of 0-9". If it sees a digit it will add that to the match. If it doesn't see a digit it still adds a 0 length string to the match. It does not mean match any string with 0 or more digits (because every string has 0 or more digits, which makes this pointless) So in your string Z, there are two zero-length matches: one at the start of the string and one at the end of the string, both of which has 0 digits.

This means that find will return true twice and matches will return false because the whole string is not a match (remember there are wo matches!).

Answer 2

because Z is not in the class [0-9]*(This should matches only the digits between 0 to 9 check the demo regex), to match Z it should be in the class [0-9Z]* (demo regex)

Pattern pt = Pattern.compile("[0-9Z]*");