The difference between Greedy Reluctant Possessive in Java regular expressions

From Java’s official documentationhttp://docs.Oracle.com/javase/7/docs/api/java/util/regex/Pattern.html we can see that there are three sets of symbols for regular expressions to represent quantifiers. They are Greedy (greedy), Reluctant (reluctant) and Possessive (exclusive). The meaning is as follows:

The difference between Greedy, Reluctant and Possessive

Examples

Looking at the table above, we find that the meanings of these three quantifiers are the same (such as X?, X??, X?+ all means once or not once), but there are still some subtle differences between them. Let’s look at an example first:

1.Greedy

public static void testGreedy() {  
    Pattern p = Pattern.compile(".*foo");  
    String strText = "xfooxxxxxxfoo";  
    Matcher m = p.matcher(strText);  
    while (m.find()) {  
        System.out.println("matched form " + m.start() + " to " + m.end());  
    }  
}

Result:

matched form 0 to 13

2.Reluctant

public static void testReluctant() {  
    Pattern p = Pattern.compile(".*?foo");  
    String strText = "xfooxxxxxxfoo";  
    Matcher m = p.matcher(strText);  
    while (m.find()) {  
        System.out.println("matched form " + m.start() + " to " + m.end());  
    }  
}

Result:

matched form 0 to 4

matched form 4 to 13

3.Possessive

public static void testPossessive() {  
    Pattern p = Pattern.compile(".*+foo");  
    String strText = "xfooxxxxxxfoo";  
    Matcher m = p.matcher(strText);  
    while (m.find()) {  
        System.out.println("matched form " + m.start() + " to " + m.end());  
    }  
}

Result:

//Unmatched successfully

Principle explanation

Greedy quantifier is called "greedy" because the matcher is forced to read the entire input string when trying to match for the first time. If If the first attempt to match fails, the character will be rolled back character by character from back to front and the match will be attempted again until the match is successful or there are no characters to fall back on.

Pattern string: .*foo

Search for string: ) position, barely reading one character at a time until the entire string is tried.

Pattern string:.*foo

Search string: , attempts a successful match once (and only once), unlike Greedy, Possessive never rolls back, and even doing so may make the overall match successful.

Pattern string: .*foo

Reference article: http://docs.oracle.com/javase /tutorial/essential/regex/quant.html

Let’s look at a few more examples:

Pattern string: .{1,9}+[0-9]

Search string: abcd5aabb6

Result: matched form 0 to 10

Pattern string: .{1,10}+[0-9]

Search string: abcd5aabb6

Result: Match failed