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Methods of the Matcher Class

This section describes how to use the methods of the Matcher class we have not yet covered. 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:

• public int start()
• public int start(int group)
• public int end()
• public int end(int group)

Study Methods

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

• public boolean lookingAt()
• public boolean find()
• public boolean find(int start)
• public boolean matches()

Replacement Methods

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

• public Matcher appendReplacement(StringBuffer sb, String replacement)
• public StringBuffer appendTail(StringBuffer sb)
• public String replaceAll(String replacement)
• public String replaceFirst(String replacement)

Using the start and end Methods

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

import java.util.regex.*;

public final class MatcherTest {

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

    public static void main(String[] argv) {
       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

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:

import java.util.regex.*;

public final 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[] argv) {
   
      // 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 occurances. Here's the ReplaceTest code:

public final class ReplaceTest {
 
    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[] argv) {
        Pattern p = Pattern.compile(REGEX);
        Matcher m = p.matcher(INPUT); // get a matcher object
        INPUT = m.replaceAll(REPLACE);
        System.out.println(INPUT);
    }
}

OUTPUT: The cat says meow. All cats say meow.

In this first version, all occurrances 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 code, ReplaceTest2:

import java.util.regex.*;
 
public final class ReplaceTest2 {
 
    private static String REGEX = "a*b";
    private static String INPUT = "aabfooaabfooabfoob";
    private static String REPLACE = "-";
 
    public static void main(String[] argv) {
        Pattern p = Pattern.compile(REGEX);
        Matcher m = p.matcher(INPUT); // get a matcher object
        INPUT = m.replaceAll(REPLACE);
        System.out.println(INPUT);
    }
}

OUTPUT: -foo-foo-foo-                                                                      

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

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

The Matcher class also provides appendReplacement and appendTail methods for text replacement. Here's an example that uses these two methods to achieve the same effect as replaceAll.

import java.util.regex.*;
 
public final class RegexTest {
 
    private static String REGEX = "a*b";
    private static String INPUT = "aabfooaabfooabfoob";
    private static String REPLACE = "-";
 
    public static void main(String[] argv) {
        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:

• public String replaceFirst(String regex, String replacement): Replaces the first substring of this string that matches the given regular expression with the given replacement. An invocation of this method of the form str.replaceFirst(regex, repl) yields exactly the same result as the expression Pattern.compile(regex).matcher(str).replaceFirst(repl)

• public String replaceAll(String regex, String replacement): Replaces each substring of this string that matches the given regular expression with the given replacement. An invocation of this method of the form str.replaceAll(regex, repl) yields exactly the same result as the expression Pattern.compile(regex).matcher(str).replaceAll(repl)

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