When Matcher#find returns false

Question

consider these two examples:

    testFind("\\W.*", "@ this is a sentence");
    testFind(".*", "@ this is a sentence");

Here's my testFind method

 private static void testFind(String regex, String input) {
    Pattern pattern = Pattern.compile(regex);
    Matcher matcher = pattern.matcher(input);
    int matches = 0;
    int nonZeroLengthMatches = 0;

    while (matcher.find()) {
        matches++;
        String matchedValue = matcher.group();
        if (matchedValue.length() > 0) {
            nonZeroLengthMatches++;
        }
        System.out.printf("Matched startIndex= %s, endIndex= %s, value: '%s'\n",
                matcher.start(), matcher.end(), matchedValue);

    }

    System.out.printf("Total non zero length matches = %s/%s \n", nonZeroLengthMatches, matches);
}

Here's the output:

 ---------------------
   Regex: '\W.*', Input: '@ this is a sentence'
   Matched startIndex= 0, endIndex= 20, value: '@ this is a sentence'
   Total non zero length matches = 1/1 
   ---------------------
   Regex: '.*', Input: '@ this is a sentence'
   Matched startIndex= 0, endIndex= 20, value: '@ this is a sentence'
   Matched startIndex= 20, endIndex= 20, value: ''
   Total non zero length matches = 1/2 

According to this: https://docs.oracle.com/javase/7/docs/api/java/util/regex/Pattern.html

Greedy quantifiers ..... X* X, zero or more times

My question is why in case of regex = "\\W.*" matcher is not giving zero-length match?

Answer 1

因为"\\W.*"意思是： "\\W" -一个非单词字符，加上".*" -零次或多次的任何字符，因此只有'@...'于此模式"\\W.*" ，但""不匹配。