Groups

Use parentheses () in the pattern to define a captured group.

    Pattern p = Pattern.compile("(\\w+)(\\s+)(\\w+)");
    String s = "These     are     spaces";
    Matcher m = p.matcher(s);

• To match a literal open parenthesis, use "(". To match a literal close parenthesis, use ")"

If a match is found, the matched text from each group is stored in the Matcher object.

• groupCount() returns the number of capturing groups in the RE.

Retrieve the string for the corresponding group with Matcher.group(int i).

• group() or group(0) returns the text matched by the entire regex.

    if (m.find()) {
      String firstWord = m.group(1);
      String spaces = m.group(2);
      String secondWord = m.group(3);
      String fullMatch = m.group();  // or m.group(0)
      System.out.println("Group 1: [" + firstWord + "]"
                     + "\nGroup 2: [" + spaces + "]"
                     + "\nGroup 3: [" + secondWord + "]"
                     + "\nFull match: [" + fullMatch + "]");
      // Group 1: [These]
      // Group 2: [     ]
      // Group 3: [are]
      // Full match: [These     are]
    }

Subsequent replacement and append methods can refer to the first matched string as $1, the second as $2, etc.

      String newString = m.replaceFirst("$3$2$1");
      System.out.println("Replaced string: " + newString);
      // Replaced string: are     These     spaces

Drill

RegularExpressionsJava/com.example.java2.drills.AddressParser

Modify your AddressParser: change the regex to capture the ZIP and area code. Modify the output to print only the ZIP and area code, not the entire line.

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