How to use regular expressions with String methods in Java?

Strings in Java have built-in support for regular expressions. 

Strings have 4 built-in methods for regular expressions, i.e., the matches(), split()), replaceFirst() and replaceAll() methods.

Method	Description
s.split("regex")	Creates an array with substrings of s divided at occurrence of "regex". "regex" is not included in the result.
s.replaceFirst("regex"), "replacement"	Replaces first occurrence of "regex" with replacement.
s.matches("regex")	Evaluates if "regex" matches s. Returns only true if the WHOLE string can be matched.
s.replaceAll("regex"), "replacement"	Replaces all occurrences of "regex" with replacement.

Now lets see the implementation of regex in Java String.

package com.codebyakram.regex;
public class RegexTest {
    public static final String DATA = "This is my small example "
            + "string which I'm going to " + "use for pattern matching.";

    public static void main(String[] args) {
        System.out.println(DATA.matches("\\w.*"));
        String[] splitString = (DATA.split("\\s+"));
        System.out.println(splitString.length);// should be 14
        for (String string : splitString) {
            System.out.println(string);
        }
        // replace all whitespace with tabs
        System.out.println(DATA.replaceAll("\\s+", "\t"));
    }
}

Examples
  Now let’s see another example for regex in Java.

package com.codebyakram.regex;;

public class StringMatcher {
    // returns true if the string matches exactly "true"
    public boolean isTrue(String s){
        return s.matches("true");
    }
    // returns true if the string matches exactly "true" or "True"
    public boolean isTrueVersion2(String s){
        return s.matches("[tT]rue");
    }

    // returns true if the string matches exactly "true" or "True"
    // or "yes" or "Yes"
    public boolean isTrueOrYes(String s){
        return s.matches("[tT]rue|[yY]es");
    }

    // returns true if the string contains exactly "true"
    public boolean containsTrue(String s){
        return s.matches(".*true.*");
    }


    // returns true if the string contains of three letters
    public boolean isThreeLetters(String s){
        return s.matches("[a-zA-Z]{3}");
        // simpler from for
//      return s.matches("[a-Z][a-Z][a-Z]");
    }



    // returns true if the string does not have a number at the beginning
    public boolean isNoNumberAtBeginning(String s){
        return s.matches("^[^\\d].*");
    }
    // returns true if the string contains a arbitrary number of characters except b
    public boolean isIntersection(String s){
        return s.matches("([\\w&&[^b]])*");
    }
    // returns true if the string contains a number less than 300
    public boolean isLessThenThreeHundred(String s){
        return s.matches("[^0-9]*[12]?[0-9]{1,2}[^0-9]*");
    }

}

What are the rules of writing regular expressions?

 Meta characters

// Removes whitespace between a word character and . or ,String pattern = "(\\w)(\\s+)([\\.,])";System.out.println(DATA.replaceAll(pattern, "$1$3"));

// Extract the text between the two title elementspattern = "(?i)(<title.*?>)(.+?)()";String updated = EXAMPLE_TEST.replaceAll(pattern, "$2");

a(?!b)