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For example if I have the text:
Sum of items is (+ 1 2 3) I want to move to the end of the line, evaluate the expression and replace it with the result, so that it reads:
Sum of items is 6 
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Typing C-x C-e in any buffer evaluates the Lisp form immediately before point and prints its value in the echo area. Typing M-: or M-x eval-expression allows you to type a Lisp form in the minibuffer which will be evaluated once you press RET.
The most general command for evaluating Lisp expressions from a buffer is eval-region. M-x eval-region parses the text of the region as one or more Lisp expressions, evaluating them one by one. M-x eval-current-buffer is similar but evaluates the entire buffer.
Emacs Lisp is a dialect of the Lisp programming language used as a scripting language by Emacs (a text editor family most commonly associated with GNU Emacs and XEmacs). It is used for implementing most of the editing functionality built into Emacs, the remainder being written in C, as is the Lisp interpreter.
Emacs Lisp supports multiple programming styles or paradigms, including functional and object-oriented. Emacs Lisp is not a purely functional programming language since side effects are common. Instead, Emacs Lisp is considered an early functional flavored language.
With the cursor at the end of the line, C-u C-x C-e will insert the value of the preceding parenthesized expression into the buffer. You could do that, then manually back up and delete the original expression. If that's too much work, here's a command that evaluates the preceding expression and replaces it with its value:
(defun replace-last-sexp () (interactive) (let ((value (eval (preceding-sexp)))) (kill-sexp -1) (insert (format "%S" value)))) Related to this, you might like Luke Gorrie's "lively.el", which provides live replacement of emacs lisp expressions within a text buffer. It's a neat hack.
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