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Thinking in Java
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Replace operations

Regular expressions become especially useful when you begin replacing text. Here are the available methods:

replaceFirst(String replacement) replaces the first matching part of the input string with replacement.

replaceAll(String replacement) replaces every matching part of the input string with replacement.

appendReplacement(StringBuffer sbuf, String replacement) performs step-by-step replacements into sbuf, rather than replacing only the first one or all of them, as in replaceFirst( ) and replaceAll( ), respectively. This is a very important method, because it allows you to call methods and perform other processing in order to produce replacement (replaceFirst( ) and replaceAll( ) are only able to put in fixed strings). With this method, you can programmatically pick apart the groups and create powerful replacements.

appendTail(StringBuffer sbuf, String replacement) is invoked after one or more invocations of the appendReplacement( ) method in order to copy the remainder of the input string.

Here’s an example that shows the use of all the replace operations. In addition, the block of commented text at the beginning is extracted and processed with regular expressions for use as input in the rest of the example:

import java.util.regex.*;
import com.bruceeckel.util.*;
import com.bruceeckel.simpletest.*;

/*! Here's a block of text to use as input to
    the regular expression matcher. Note that we'll
    first extract the block of text by looking for
    the special delimiters, then process the
    extracted block. !*/

public class TheReplacements {
  private static Test monitor = new Test();
  public static void main(String[] args) throws Exception {
    String s ="");
    // Match the specially-commented block of text above:
    Matcher mInput =
      Pattern.compile("/\\*!(.*)!\\*/", Pattern.DOTALL)
      s =; // Captured by parentheses
    // Replace two or more spaces with a single space:
    s = s.replaceAll(" {2,}", " ");
    // Replace one or more spaces at the beginning of each
    // line with no spaces. Must enable MULTILINE mode:
    s = s.replaceAll("(?m)^ +", "");
    s = s.replaceFirst("[aeiou]", "(VOWEL1)");
    StringBuffer sbuf = new StringBuffer();
    Pattern p = Pattern.compile("[aeiou]");
    Matcher m = p.matcher(s);
    // Process the find information as you
    // perform the replacements:
    // Put in the remainder of the text:
    monitor.expect(new String[]{
      "Here's a block of text to use as input to",
      "the regular expression matcher. Note that we'll",
      "first extract the block of text by looking for",
      "the special delimiters, then process the",
      "extracted block. ",
      "H(VOWEL1)rE's A blOck Of tExt tO UsE As InpUt tO",
      "thE rEgUlAr ExprEssIOn mAtchEr. NOtE thAt wE'll",
      "fIrst ExtrAct thE blOck Of tExt by lOOkIng fOr",
      "thE spEcIAl dElImItErs, thEn prOcEss thE",
      "ExtrActEd blOck. "
} ///:~

The file is opened and read using the ) method introduced earlier in this chapter. mInput is created to match all the text (notice the grouping parentheses) between ‘/*!’ and ‘!*/’. Then, more than two spaces are reduced to a single space, and any space at the beginning of each line is removed (in order to do this on all lines and not just the beginning of the input, multiline mode must be enabled). These two replacements are performed with the equivalent (but more convenient, in this case) replaceAll( ) that’s part of String. Note that since each replacement is only used once in the program, there’s no extra cost to doing it this way rather than precompiling it as a Pattern.

replaceFirst( ) only performs the first replacement that it finds. In addition, the replacement strings in replaceFirst( ) and replaceAll( ) are just literals, so if you want to perform some processing on each replacement they don’t help. In that case, you need to use appendReplacement( ), which allows you to write any amount of code in the process of performing the replacement. In the preceding example, a group( ) is selected and processed—in this situation, setting the vowel found by the regular expression to upper case—as the resulting sbuf is being built. Normally, you would step through and perform all the replacements and then call appendTail( ), but if you wanted to simulate replaceFirst( ) (or “replace n”), you would just do the replacement one time and then call appendTail( ) to put the rest into sbuf.

appendReplacement( ) also allows you to refer to captured groups directly in the replacement string by saying “$g” where ‘g’ is the group number. However, this is for simpler processing and wouldn’t give you the desired results in the preceding program.
Thinking in Java
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire