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Following is a simple program demonstrating how to use regular expression in Java. Java Regex Characters

Java Regex Capturing Groups

Java Regex Capturing Groups

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".Capturing groups are numbered by counting their opening parentheses from the left to the right.In the expression ((A)(B(C))), for example, there are four such groups -((A)(B(C)))(A)(B(C))(C)Back references allow repeating a capturing group using a number like \# where # is the groupnumber. See the example below −ExampleLive Demoimport java.util.regex.Matcher; import java.util.regex.Pattern; public class Tester {   ... Read More

https://www.tutorialspoint.com/javaregex/javaregex_capturing_groups.htm

ExampleLive Demoimport java.util.regex.Matcher; import java.util.regex.Pattern; public class NoRegTest {    public static void main(String[] args) {       String s="^fun";       Pattern pattern = Pattern.compile(s);       Matcher matcher = pattern.matcher("Java is fun");       if(!matcher.find()) {          System.out.println("not found");       }    } }Outputnot found

regex ^((?!kk).)*$ returns true if a line does not contain kk, otherwise returns falseExamplepublic class RegTest {    public static void main(String[] args) {       // TODO Auto-generated method stub       String s="tutorials";       boolean i=s.matches("^((?!kk).)*$");       System.out.println(i);    } }

Exampleimport java.util.regex.*; class PatternMatch{    public static void main(String args[]) {       String content = "I am a student";       String string = ".*boy.*";       boolean isMatch = Pattern.matches(string, content);       System.out.println("The line contains 'boy'?"+ isMatch);    } }Outputthe line contains 'boy'?falsematches()­­It is used to check if the whole text matches a pattern. Its output is boolean. It returns true if match is found otherwise false.This is one of simplest and easiest way of searching a String in a text using Regex .There is a another method compile() , if you want ... Read More

\R matches any line break as defined by the Unicode standardPattern p = Pattern.compile("\R");Unicode line-break sequence is equivalent to \u000D\u000A|[\u000A\u000B\u000C\u000D\u0085\u2028\u2029]

In Java regex matches() matches the input string against the whole string as it add a ^ and $ at the end of the input string.so it will not match the substring. So for matching substring, you should use find().Exampleimport java.util.regex.*; class PatternMatchingExample {    public static void main(String args[]) {       String content = "aabbcc";       String string = "aa";       Pattern p = Pattern.compile(string);       Matcher m = p.matcher(content);       System.out.println(" 'aa' Match:"+ m.matches());       System.out.println(" 'aa' Match:"+ m.find());    } }Output'aa' Match:false 'aa' Match:true