input = input.replaceAll("\\(\\)|\\[\\]|\\{\\}", ""))

This will replace ()[]{} with "" , making input length lesser than earlier

Now this *input.length()* will be the length of ()[]{} removed input

So, if you start with ([{}])...

input.length() is 6 , after input.replaceAll(..), *input* will be '([])' , and its length is 4 where *6!=4*,so while loop continues

1. '([{}])' != '([])' <-- result of first replaceAll
2. '([])' != '()' <-- result of second
3. '()' != '' <-- result of third
4. '' == ''
5. and isEmpty() returns true