Quantifiers in Java

06 Dec, 2021
Prerequisite: Regular Expressions in Java
Quantifiers in Java allow users to specify the number of occurrences to match against. Below are some commonly used quantifiers in Java.
X*        Zero or more occurrences of X
X?        Zero or One occurrences of X
X+        One or More occurrences of X
X{n}      Exactly n occurrences of X 
X{n, }    At-least n occurrences of X
X{n, m}   Count of occurrences of X is from n to m
The above quantifiers can be made Greedy, Reluctant, and Possessive.
By default, quantifiers are Greedy. Greedy quantifiers try to match the longest text that matches a given pattern. Greedy quantifiers work by first reading the entire string before trying any match. If the whole text doesn’t match, remove the last character and try again, repeating the process until a match is found.  
Java
// Java program to demonstrate Greedy Quantifiers import java.util.regex.Matcher;import java.util.regex.Pattern;  class Test{    public static void main(String[] args)    {        // Making an instance of Pattern class        // By default quantifier "+" is Greedy        Pattern p = Pattern.compile("g+");          // Making an instance of Matcher class        Matcher m = p.matcher("ggg");          while (m.find())            System.out.println("Pattern found from " + m.start() +                               " to " + (m.end()-1));      }}
Pattern found from 0 to 2
Explanation: The pattern g+ means one or more occurrences of g. Text is ggg. The greedy matcher would match the longest text even if parts of the matching text also match. In this example, g and gg also match, but the greedy matcher produces ggg.
This quantifier uses the approach that is the opposite of greedy quantifiers. It starts with the first character and processes one character at a time. 
Java
// Java program to demonstrate Reluctant Quantifiers import java.util.regex.Matcher;import java.util.regex.Pattern;  class Test{    public static void main(String[] args)    {        // Making an instance of Pattern class        // Here "+" is a Reluctant quantifier because        // a "?' is appended after it.        Pattern p = Pattern.compile("g+?");          // Making an instance of Matcher class        Matcher m = p.matcher("ggg");          while (m.find())            System.out.println("Pattern found from " + m.start() +                               " to " + (m.end()-1));      }}
Pattern found from 0 to 0
Pattern found from 1 to 1
Pattern found from 2 to 2
Explanation: Since the quantifier is reluctant, it matches the shortest part of the test with the pattern. It processes one character at a time.
This quantifier matches as many characters as possible, like a greedy quantifier. But if the entire string doesn’t match, then it doesn’t try removing characters from the end.
Java
// Java program to demonstrate Possessive Quantifiers import java.util.regex.Matcher;import java.util.regex.Pattern;  class Test{    public static void main(String[] args)    {        // Making an instance of Pattern class        // Here "+" is a Possessive quantifier because        // a "+' is appended after it.        Pattern p = Pattern.compile("g++");          // Making an instance of Matcher class        Matcher m = p.matcher("ggg");          while (m.find())            System.out.println("Pattern found from " + m.start() +                               " to " + (m.end()-1));    }}
Pattern found from 0 to 2
Explanation: We get the same output as Greedy because the whole text matches the pattern.
Java
// Java program to demonstrate difference// between Possessive and Greedy Quantifiers import java.util.regex.Matcher;import java.util.regex.Pattern;  class Test{    public static void main(String[] args)    {        // Create a pattern with Greedy quantifier        Pattern pg = Pattern.compile("g+g");         // Create same pattern with possessive quantifier        Pattern pp = Pattern.compile("g++g");                 System.out.println("Using Greedy Quantifier");        Matcher mg = pg.matcher("ggg");        while (mg.find())            System.out.println("Pattern found from " + mg.start() +                               " to " + (mg.end()-1));         System.out.println("\nUsing Possessive Quantifier");        Matcher mp = pp.matcher("ggg");        while (mp.find())            System.out.println("Pattern found from " + mp.start() +                               " to " + (mp.end()-1));     }}
Using Greedy Quantifier
Pattern found from 0 to 2

Using Possessive Quantifier
In the above example, since the first quantifier is greedy, g+ matches the whole string. If we match g+ with whole string, g+g doesn’t match, the Greedy quantifier removes the last character, matches gg with g+, and finds a match. In the Possessive quantifier, we start like Greedy. g+ matches the whole string, but matching g+ with the whole string doesn’t match g+g with ggg. Unlike Greedy, since quantifier is possessive, we stop at this point.
This article is contributed by Rahul Agarwal. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above
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