Why does replaceAll fail with "illegal group reference"?

Question

I am in need to replace

\\\s+\\$\\$ to $$ 

I used

String s = "  $$"; s = s.replaceAll("\\s+\\$\\$","$$"); 

but it throws exception

java.lang.IllegalArgumentException: Illegal group reference

Answer 1

From String#replaceAll javadoc:

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 Matcher.quoteReplacement(java.lang.String) to suppress the special meaning of these characters, if desired.

So escaping of an arbitrary replacement string can be done using Matcher#quoteReplacement:

String s = "  $$"; s = s.replaceAll("\\s+\\$\\$", Matcher.quoteReplacement("$$")); 

Also escaping of the pattern can be done with Pattern#quote

String s = "  $$"; s = s.replaceAll("\\s+" + Pattern.quote("$$"), Matcher.quoteReplacement("$$"));