Capture Groups

A Pattern with capture groups can be used to construct a Matcher. After the Matcher has done pattern matching, the characters in each group are available using its group(int number) method. Examine the following code:

String response = "123";  // Assume this data came from the user

Pattern numPat    = Pattern.compile( "(\\d)(\\d)(\\d)"  );    
Matcher numMatch  = numPat.matcher( response ); 

if ( numMatch.matches() )
{
  System.out.print  ("Digit 1: " + numMatch.group(1) + ", ");   
  System.out.print  ("Digit 2: " + numMatch.group(2) + ", " );   
  System.out.println("Digit 3: " + numMatch.group(3) );   
}
else
  System.out.println("No Match." );
}
 

The program prints:

Digit 1: 1, Digit 2: 2, Digit 3: 3

group(0) corresponds to the entire pattern that was matched (in this program, the entire string "123"). group(1) corresponds to the first capture group (that matches "1" in this program). group(2) corresponds to the first capture group (that matches "2" in this program), and so on.

• If a group fails to match part of the input string, it is set to null.
• If a group matches the empty string, then it is set to the empty string.
• If the Matcher fails to match the input string (so that matches() returns false), then group() can not be used.