Java SE: String.replaceAll() with special character "$"

1. Problem Description:

    When replacement contains "$", either IllegalArgumentException or StringIndexOutOfBoundsException will be thrown.

    The reason is that replaceAll() is not simply character replacement, it uses regex as "placeholder expression".

    And the character "$" is treated as the end of line boundry matcher in regex.

 

    Java API for replaceAll() method:

public String replaceAll(String regex, String replacement);

    JavaDoc For replaceAll() method:

Note that backslashes (\) and dollar signs ($) in the replacement string may cause the results to be different than if it were being treated as a literal replacement string; see Matcher.replaceAll. Use java.util.regex.Matcher.quoteReplacement to suppress the special meaning of these characters, if desired.

 

    Case 1:

package edu.xmu.string;

public class ReplaceAllTest {
    public static void main(String[] args) {
	String str = "abcdefg";
	String placeholder = "abc";
	String replacement = "a$bc";
	String replacedStr = str.replaceAll(placeholder, replacement);
	System.out.println(replacedStr);
    }
}

Exception in thread "main" java.lang.IllegalArgumentException: Illegal group reference
	at java.util.regex.Matcher.appendReplacement(Matcher.java:808)
	at java.util.regex.Matcher.replaceAll(Matcher.java:906)
	at java.lang.String.replaceAll(String.java:2162)
	at edu.xmu.string.ReplaceAllTest.main(ReplaceAllTest.java:8)

    Case 2:

package edu.xmu.string;

public class ReplaceAllTest {
    public static void main(String[] args) {
	String str = "abcdefg";
	String placeholder = "abc";
	String replacement = "abc$";
	String replacedStr = str.replaceAll(placeholder, replacement);
	System.out.println(replacedStr);
    }
}

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 4
	at java.lang.String.charAt(String.java:658)
	at java.util.regex.Matcher.appendReplacement(Matcher.java:772)
	at java.util.regex.Matcher.replaceAll(Matcher.java:906)
	at java.lang.String.replaceAll(String.java:2162)
	at edu.xmu.string.ReplaceAllTest.main(ReplaceAllTest.java:8)

 

2) Available Solutions:

    1) If we may predefine (hardcode) the replacement:

package edu.xmu.string;

public class ReplaceAllTest {
    public static void main(String[] args) {
	String str = "abcdefg";
	String placeholder = "abc";
	String replacement = "\\$abc"; // Use backslash to make sure "$" is treated as normal character
	String replacedStr = str.replaceAll(placeholder, replacement);
	System.out.println(replacedStr);
    }
}

    2) If we cannot predefine (hardcode) the replacement(the replacement is passed in as parameter):

    Manually preprocess for special character in replacement. 

package edu.xmu.string;

public class ReplaceAllTest {
    public static void main(String[] args) {
	String str = "abcdefg";
	String placeholder = "abc";
	String replacement = "$abc";
	replacement = replacement.replaceAll("\\$", "\\\\\\$"); // preprocessment for the special character in replacement.
	String replacedStr = str.replaceAll(placeholder, replacement);
	System.out.println(replacedStr);
    }
}

    Or automatically preprocess for special character in replacement.

package edu.xmu.string;

import java.util.regex.Matcher;

public class ReplaceAllTest {
    public static void main(String[] args) {
	String str = "abcdefg";
	String placeholder = "abc";
	String replacement = "$abc";
	replacement = Matcher.quoteReplacement(replacement);  // preprocessment for the special character in replacement.
	String replacedStr = str.replaceAll(placeholder, replacement);
	System.out.println(replacedStr);
    }
}

   Or use Apache's StringUtils.replace(...), it is preferred if all we need is simple character replacement without using regex. 

package edu.xmu.string;

import org.apache.commons.lang.StringUtils;

public class ReplaceAllTest {
    public static void main(String[] args) {
	String str = "abcdefg";
	String placeholder = "abc";
	String replacement = "$abc";
	String replacedStr = StringUtils.replace(str, placeholder, replacement); // apache's StringUtils do not use regex for replacement.
	System.out.println(replacedStr);
    }
}

 

Reference Links:

1) http://www.coderanch.com/t/383666/java/java/String-replaceAll