Java: Understanding the String replaceAll() method

Question

I'm looking to figure out the answer to this problem here.

First off,

blah[abc] = blah[abc].replaceAll("(.*) (.*)", "$2, $1");

Can someone explain to me what the (.*), $2 and $1 are?

Secondly, when I nest that within a for statement in order to reverse two parts of a string, I am hit with an exception error. I was wondering if anybody knew why that is.

Thanks

Edit: This is the error I receive

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 1 at ChangeNames.main(ChangeNames.java:21)

Answer 1

(.*) - would be a pattern to match any number of characters. Parentheses would be to mark it as a sub pattern (for back reference).

$2 & $1 - are back references. These would be things matched in your second and first sub pattern.

Basically replaceAll("(.) (.)", "$2, $1") would find characters separated by a space, then add a comma before the space, in addition to flipping the parts. For example:

a b => b, a
Hello world => Hellw, oorld

Not sure about nesting... Can you post the code you're running?