Question

I don’t understand why the “$” is special.

String str = "bla aa";
String tag = "$";
str = str.replaceFirst("aa", tag);

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 1

If I change the tag = “\\$”, then it works fine. But why does it need to be escaped? thanks in advance.

Answer 1

Because it is a special regex symbol (in results it’s about capturing groups), and replaceFirst takes regex arguments. The documentation explicitly warns you:

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

Now a bit more about $. In the regex pattern it means “end of line”.
In the replacement string, $g means “the g th group”. So for a regex a([a-z]+)([0-9]+), you have two groups – $1 and $2, and you can refer to them when replacing. See the explanation here