Trail: Essential Classes
Lesson: Regular Expressions
Methods of the Matcher Class
Home Page > Essential Classes > Regular Expressions

Methods of the Matcher Class

This section describes some additional useful methods of the Matcher class. For convenience, the methods listed below are grouped according to functionality.

Index Methods

Index methods provide useful index values that show precisely where the match was found in the input string:

Study Methods

Study methods review the input string and return a boolean indicating whether or not the pattern is found.

Replacement Methods

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

Using the start and end Methods

Here's an example, MatcherDemo.java, that counts the number of times the word "dog" appears in the input string.


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

public class MatcherDemo {

    private static final String REGEX =
        "\\bdog\\b";
    private static final String INPUT =
        "dog dog dog doggie dogg";

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

You can see that this example uses word boundaries to ensure that the letters "d" "o" "g" are not merely a substring in a longer word. It also gives some useful information about where in the input string the match has occurred. The start method returns the start index of the subsequence captured by the given group during the previous match operation, and end returns the index of the last character matched, plus one.

Using the matches and lookingAt Methods

The matches and lookingAt methods both attempt to match an input sequence against a pattern. The difference, however, is that matches requires the entire input sequence to be matched, while lookingAt does not. Both methods always start at the beginning of the input string. Here's the full code, MatchesLooking.java:


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

public class MatchesLooking {

    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) {
   
        // Initialize
        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());
    }
}

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

Using replaceFirst(String) and replaceAll(String)

The replaceFirst and replaceAll methods replace text that matches a given regular expression. As their names indicate, replaceFirst replaces the first occurrence, and replaceAll replaces all occurences. Here's the ReplaceDemo.java code:


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

public class ReplaceDemo {
 
    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.

In this first version, all occurrences of dog are replaced with cat. But why stop here? Rather than replace a simple literal like dog, you can replace text that matches any regular expression. The API for this method states that "given the regular expression a*b, the input aabfooaabfooabfoob, and the replacement string -, an invocation of this method on a matcher for that expression would yield the string -foo-foo-foo-."

Here's the ReplaceDemo2.java code:


import java.util.regex.Pattern;
import java.util.regex.Matcher;
 
public class ReplaceDemo2 {
 
    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);
        INPUT = m.replaceAll(REPLACE);
        System.out.println(INPUT);
    }
}

OUTPUT: -foo-foo-foo-

To replace only the first occurrence of the pattern, simply call replaceFirst instead of replaceAll. It accepts the same parameter.

Using appendReplacement(StringBuffer,String) and appendTail(StringBuffer)

The Matcher class also provides appendReplacement and appendTail methods for text replacement. The following example, RegexDemo.java, uses these two methods to achieve the same effect as replaceAll.


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

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


OUTPUT: -foo-foo-foo- 

Matcher Method Equivalents in java.lang.String

For convenience, the String class mimics a couple of Matcher methods as well:


Problems with the examples? Try Compiling and Running the Examples: FAQs.
Complaints? Compliments? Suggestions? Give us your feedback.

Previous page: Methods of the Pattern Class
Next page: Methods of the PatternSyntaxException Class



Spec-Zone.ru - all specs in one place