I would like to find replace this in eclipse:
Try this. You have to make sure you have regular expressions turned on in the Find/Replace window.
Find: \$_GET\[('.*')\] Replace: \$this->input->get($1)
It's not clear to me what your 'whatever' needs to be. If it's only word characters you can replace the .* with \w. You have to precede the dollar sign and the left and right brackets with a backslash (escape them) because they are special characters in the regex and if not escaped would be treated differently by the regex parser.