How do I use possessive quantifier regex?

The possessive quantifiers always eat the entire input string, trying once (and only once) for a match. Unlike the greedy quantifiers, possessive quantifiers never back off, even if doing so would allow the overall match to succeed.

package org.kodejava.example.util.regex;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class PossessiveQuantifierDemo {
    public static void main(String[] args) {
        String[] regexs = {
                "x?+",
                "x*+",
                "x++",
                "x{2}+",
                "x{2,}+",
                "x{2,5}+"
        };
        String input = "xxxxxxx";

        for (String r : regexs) {
            Pattern pattern = Pattern.compile(r);
            Matcher matcher = pattern.matcher(input);

            //
            // Find every match and print it
            //
            System.out.println("------------------------------");
            System.out.format("Regex:  %s %n", r);
            while (matcher.find()) {
                System.out.format("Text "%s" found at %d to %d.%n",
                        matcher.group(), matcher.start(), 
                        matcher.end());
            }
        }
    }
}

How do I write embedded flag expression?

It’s also possible to enable various flags using embedded flag expressions. Embedded flag expressions are an alternative to the two-argument version of compile, and are specified in the regular expression itself. The example below is use (?i) flag expression to enable case-insensitive matching.

Another flag expressions are listed below:

  • (?x), equivalent with Pattern.COMMENTS
  • (?m), equivalent with Pattern.MULTILINE
  • (?s), equivalent with Pattern.DOTTAL
  • (?u), equivalent with Pattern.UNICODE_CASE
  • (?d), equivalent with Pattern.UNIX_LINES

 

package org.kodejava.example.util.regex;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class EmbeddedFlagDemo {
    public static void main(String[] args) {
        //
        // Define regex which starting with (?i) to enable 
        // case-insensitive matching
        //
        String regex = "(?i)the";
        String text = "The quick brown fox jumps over the lazy dog";

        //
        // Obtain the required matcher
        //
        Pattern pattern = Pattern.compile(regex);        
        Matcher matcher = pattern.matcher(text);

        //
        // Find every match and print it
        //
        while (matcher.find()) {
            System.out.format("Text "%s" found at %d to %d.%n",
                    matcher.group(), matcher.start(), 
                    matcher.end());
        }
    }
}

The result of the program is:

Text "The" found at 0 to 3.
Text "the" found at 31 to 34.

How do I use quantifier in regex?

A quantifier following a subsequence of a pattern determines the possibilities for how that subsequence of a pattern can repeat. Quantifiers allow you to specify the number of occurrences to match against.

Quantifiers

  • X? : X, once or not at all
  • X* : X, zero or more times
  • X+ : X, one or more times
  • X{n} : X, exactly n times
  • X{n,} : X, at least n times
  • X{n,m} : X, at least n but not more than m times
package org.kodejava.example.util.regex;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class RegexQuantifierDemo {
    public static void main(String[] args) {
        String[] expressions =
                {"x?", "x*", "x+", "x{2}", "x{2,}", "x{2,5}"};

        String input = "xxxxxx yyyxxxxxx zzzxxxxxx";

        for (String expression : expressions) {
            // Compiles the given regular expression into a
            // pattern and creates a matcher that will match
            // the given input against this pattern.
            Pattern pattern = Pattern.compile(expression);
            Matcher matcher = pattern.matcher(input);

            // Find every match and print it
            System.out.println("------------------------------");
            System.out.format("regex:  %s %n", expression);
            while (matcher.find()) {
                System.out.format("Text \"%s\" found at %d to %d%n",
                        matcher.group(), matcher.start(),
                        matcher.end());
            }
        }
    }
}

Here are the result of the program:

------------------------------
regex:  x? 
Text "x" found at 0 to 1
Text "x" found at 1 to 2
Text "x" found at 2 to 3
Text "x" found at 3 to 4
Text "x" found at 4 to 5
Text "x" found at 5 to 6
Text "" found at 6 to 6
Text "" found at 7 to 7
Text "" found at 8 to 8
Text "" found at 9 to 9
Text "x" found at 10 to 11
Text "x" found at 11 to 12
Text "x" found at 12 to 13
Text "x" found at 13 to 14
Text "x" found at 14 to 15
Text "x" found at 15 to 16
Text "" found at 16 to 16
Text "" found at 17 to 17
Text "" found at 18 to 18
Text "" found at 19 to 19
Text "x" found at 20 to 21
Text "x" found at 21 to 22
Text "x" found at 22 to 23
Text "x" found at 23 to 24
Text "x" found at 24 to 25
Text "x" found at 25 to 26
Text "" found at 26 to 26
------------------------------
regex:  x* 
Text "xxxxxx" found at 0 to 6
Text "" found at 6 to 6
Text "" found at 7 to 7
Text "" found at 8 to 8
Text "" found at 9 to 9
Text "xxxxxx" found at 10 to 16
Text "" found at 16 to 16
Text "" found at 17 to 17
Text "" found at 18 to 18
Text "" found at 19 to 19
Text "xxxxxx" found at 20 to 26
Text "" found at 26 to 26
------------------------------
regex:  x+ 
Text "xxxxxx" found at 0 to 6
Text "xxxxxx" found at 10 to 16
Text "xxxxxx" found at 20 to 26
------------------------------
regex:  x{2} 
Text "xx" found at 0 to 2
Text "xx" found at 2 to 4
Text "xx" found at 4 to 6
Text "xx" found at 10 to 12
Text "xx" found at 12 to 14
Text "xx" found at 14 to 16
Text "xx" found at 20 to 22
Text "xx" found at 22 to 24
Text "xx" found at 24 to 26
------------------------------
regex:  x{2,} 
Text "xxxxxx" found at 0 to 6
Text "xxxxxx" found at 10 to 16
Text "xxxxxx" found at 20 to 26
------------------------------
regex:  x{2,5} 
Text "xxxxx" found at 0 to 5
Text "xxxxx" found at 10 to 15
Text "xxxxx" found at 20 to 25

How do I use capturing groups in regex?

Capturing groups are a way to treat multiple characters as a single unit. They are created by placing the characters to be grouped inside a set of parentheses. For example, the regular expression (dog) creates a single group containing the letters d, o and g.

Regular expressions can also define other capturing groups that correspond to parts of the pattern. Each pair of parentheses in a regular expression defines a separate capturing group in addition to the group that the whole expression defines.

package org.kodejava.example.util.regex;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class CapturingGroupDemo {
    public static void main(String[] args) {
        // Define regex to find the word 'the' or 'quick'
        String regex = "(the)|(quick)";
        String text = "the quick brown fox jumps over the lazy dog";

        // Compiles the given regular expression into a pattern and
        // Creates a matcher that will match the given input against
        // this pattern.
        Pattern pattern = Pattern.compile(regex);
        Matcher matcher = pattern.matcher(text);

        // Find every match and print it
        while (matcher.find()) {
            System.out.format("Text \"%s\" found at %d to %d.%n",
                    matcher.group(), matcher.start(), matcher.end());
        }
    }
}

The results of the program are:

Text "the" found at 0 to 3.
Text "quick" found at 4 to 9.
Text "the" found at 31 to 34.

How do I use logical OR operator in regex?

You can use the | operator (logical OR) to match characters or expression of either the left or right of the | operator. For example the [t|T] will match either t or T from the input string.

package org.kodejava.example.util.regex;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class LogicalOrRegexDemo {
    public static void main(String[] args) {
        //
        // Define regex that will search characters 't' or 'T'
        //
        String regex = "[t|T]";

        //
        // Compiles the pattern and obtains the matcher object.
        //
        Pattern pattern = Pattern.compile(regex);
        Matcher matcher = pattern.matcher(
                "The quick brown fox jumps over the lazy dog");

        //
        // Find every match and print it
        //
        while (matcher.find()) {
            System.out.format("Text "%s" found at %d to %d.%n",
                    matcher.group(), matcher.start(), matcher.end());
        }
    }
}

The program print the following result:

Text "T" found at 0 to 1.
Text "t" found at 31 to 32.