JAVA: illegal group reference

How:
You have a string "I like dollar symbol".
and you want to replace the string with "I like $ symbol".
Code:

String a = "I like dollar symbol"
String replacement = "$"
a = a.replaceAll("dollar", replacement);

This will throw an exception "Illegal group reference."

java.lang.IllegalArgumentException: Illegal group reference
at java.util.regex.Matcher.appendReplacement(Matcher.java:713)
at java.util.regex.Matcher.replaceAll(Matcher.java:813)
at java.lang.String.replaceAll(String.java:2190)

Solution:
Here the problem is "$" is a Metacharacter which should be escaped.

• replacement = Matcher.quoteReplacement(replacement);
  a = a.replaceAll("dollar", replacement);
  

  
  About quoteReplacement method in Matcher class
  Ref. link: Click here
  Returns a literal replacement String for the specified String. This method produces a String that will work use as a literal replacement s in the appendReplacement method of the Matcher class. The String produced will match the sequence of characters in s treated as a literal sequence. Slashes ('\') and dollar signs ('$') will be given no special meaning.
  


• put two backslashes before "$".

  String replacement = "\\$"