String.replaceAll() throwing IndexOutOfBoundsException

Question

I get java.lang.IndexOutOfBoundsException, when I call replaceAll() with replacement text containing $1:

This is from a more complex code, but I simplified it as follows:

http://ideone.com/QCof3

class Test {
    public static void main(String args[]) {
        String test = "@Key";
        String replacement = "$1";
        test = test.replaceAll("@Key", replacement);
        System.out.println(test);
    }
}


Exception in thread "main" java.lang.IndexOutOfBoundsException: No group 1
    at java.util.regex.Matcher.group(Matcher.java:470)
    at java.util.regex.Matcher.appendReplacement(Matcher.java:737)
    at java.util.regex.Matcher.replaceAll(Matcher.java:813)
    at java.lang.String.replaceAll(String.java:2189)
    at Test.main(Main.java:5)

Are there any workarounds for this issue? I do not want to use a 3rd party library.

Answer 1

try with the replace() only

test = test.replace("@Key", replacement);

Answer 2

String replacement = "\\$1";

should resolve it. $ is a regex control character, so just like any other control character it must be escaped. Java being java, the escape has to be a double backslash.

Answer 3

If you really need the regex in String.replaceAll() instead of just the String.replace(), which really does replace all strings but just does not use regex, you can use Matcher.quoteReplacement()

String stringwithgroup = "Give me all your $$$$";
String result = proposal.replaceAll("Marry me",Matcher.quoteReplacement(stringwithgroup));

Answer 4

    String test = "@Key";
    String replacement = "\\$1";
    test = test.replaceAll("@Key", replacement);
    System.out.println(test);

---

• IDEOne Demo

Answer 5

Check your regular expression. You want to catch the first group that was selected in your regular expression. But your regular expression is only $1. This means that it takes the first group. But there is no such group, so there is the exception.

 Exception in thread "main" java.lang.IndexOutOfBoundsException: No group 1

Try using this command with this regular expression.

Class Test {  
   public static void main(String args[]) {  
      String test = "@Key";  
      String replacement = "!!$1";  
      test = test.replaceAll("(@Key)", replacement);  
      System.out.println(test);  
   }  
}

The result is test = !!@Key. Because the first group is @Key and replacement by !!@Key.

Please check there links for rugular expressions. Lesson REGEX
And: Search and replace with regular expressions

Hope this help

Answer 6

Solution

This is what you need to do:

String test = "@Key";
String replacement = "\\$1";
test = test.replaceAll("@Key", replacement);
System.out.println(test);

Use of $#

'$#', '#' being a number in the second argument of replaceAll means it will get the 1st group of matches. So to use it correctly, here's an example:

String test = "World Hello!!!";
String replacement = "$2 $1";
test = test.replaceAll("(World) (Hello)", replacement);
System.out.println(test);

You create groups with the brackets, and the code take the two groups and swap them around.

Use of $

'$' in regular expression means the end of the line. So by using '$' as the first argument in replaceAll, will append to the end of the string, ie:

String test = "World Hello!!!";     
test = test.replaceAll("$", " ~ a java dev");
System.out.println(test);