java tutorial - Java - Regular Expressions - java programming - learn java - java basics - java for beginners

The java.util.regex package to be begin with following three classes:

Collecting groups are numbered by determining the number of opening parentheses from left to right. Thus, in the expression ((A) (B (C))) there are four similar groups:

Below is an example of a regular expression in Java, illustrating the way to identify a string of digits in alphanumeric strings.

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

public class RegexMatches {

   public static void main( String args[] ) {
      // String to be scanned to find the pattern.
      String line = "This order was placed for QT3000! OK?";
      String pattern = "(.*)(\\d+)(.*)";

      // Create a Pattern object
      Pattern r = Pattern.compile(pattern);

      // Now create matcher object.
      Matcher m = r.matcher(line);
      if (m.find( )) {
         System.out.println("Found value: " + m.group(0) );
         System.out.println("Found value: " + m.group(1) );
         System.out.println("Found value: " + m.group(2) );
      }else {
         System.out.println("NO MATCH");
      }
   }
}

Output

Found value: This order was placed for QT3000! OK?
Found value: This order was placed for QT300
Found value: 0

In Java, regular expressions use special characters. The following table shows the metacharacters available in the regular expression syntax.

Index methods represent useful index values ​​that show the exact number of matches found in the input string.

The methods of the analysis perform an analysis of the input string and return a Boolean value indicating the presence or absence of a template.

Replacement methods provide useful methods for replacing text in an input string.

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

public class RegexMatches {

   private static final String REGEX = "\\bcat\\b";
   private static final String INPUT = "cat cat cat cattie cat";

   public static void main( String args[] ) {
      Pattern p = Pattern.compile(REGEX);
      Matcher m = p.matcher(INPUT);   // get a matcher object
      int count = 0;

      while(m.find()) {
         count++;
         System.out.println("Match number "+count);
         System.out.println("start(): "+m.start());
         System.out.println("end(): "+m.end());
      }
   }
}

Output

Match number 1
start(): 0
end(): 3
Match number 2
start(): 4
end(): 7
Match number 3
start(): 8
end(): 11
Match number 4
start(): 19
end(): 22

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

public class RegexMatches {

   private static final String REGEX = "foo";
   private static final String INPUT = "fooooooooooooooooo";
   private static Pattern pattern;
   private static Matcher matcher;

   public static void main( String args[] ) {
      pattern = Pattern.compile(REGEX);
      matcher = pattern.matcher(INPUT);

      System.out.println("Current REGEX is: "+REGEX);
      System.out.println("Current INPUT is: "+INPUT);

      System.out.println("lookingAt(): "+matcher.lookingAt());
      System.out.println("matches(): "+matcher.matches());
   }
}

the following result will be obtained:

Current REGEX is: foo
Current INPUT is: fooooooooooooooooo
lookingAt(): true
matches(): false

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

public class RegexMatches {

   private static String REGEX = "dog";
   private static String INPUT = "The dog says meow. " + "All dogs say meow.";
   private static String REPLACE = "cat";

   public static void main(String[] args) {
      Pattern p = Pattern.compile(REGEX);
      
      // get a matcher object
      Matcher m = p.matcher(INPUT); 
      INPUT = m.replaceAll(REPLACE);
      System.out.println(INPUT);
   }
}

Output

The cat says meow. All cats say meow.

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

public class RegexMatches {

   private static String REGEX = "a*b";
   private static String INPUT = "aabfooaabfooabfoob";
   private static String REPLACE = "-";
   public static void main(String[] args) {

      Pattern p = Pattern.compile(REGEX);
      
      // get a matcher object
      Matcher m = p.matcher(INPUT);
      StringBuffer sb = new StringBuffer();
      while(m.find()) {
         m.appendReplacement(sb, REPLACE);
      }
      m.appendTail(sb);
      System.out.println(sb.toString());
   }
}

Output

-foo-foo-foo-