Selecting things in Emacs

You can select blocks of text in Emacs just as you would in most other environments. You could, for example, drag your mouse over a region. You could also hold down the Shift key and use arrow keys. But Emacs also has a number of commands that let you work in larger semantic units. That is, instead of working with an undifferentiated set of characters, you can select meaningful chunks of text, the meaning depending on context.

When you’re editing English prose, the semantic units you are concerned with might be words, sentences, or paragraphs. When you’re editing programming language source code, you care about functions or various “balanced expressions” such as the content between two parentheses or two curly brackets.

The following table gives some of the selection commands built into Emacs.

UnitCommandKey binding
wordmark-wordM-@
paragraphmark-paragraphM-h
pagemark-pageC-x C-p
buffermark-whole-buffer C-x h
functionmark-defunC-M-h
balanced expressionmark-sexpC-M-@

The expand-region package offers an alternative to several of these commands. More on that later.

The command for selecting a word does just what you expect. Likewise, the commands for selecting a page or a buffer require little explanation. But the meaning of a “paragraph” depends on context (i.e. editing mode), as do the meanings of “function” and “balanced expression.”

When editing source code, a “paragraph” is typically a block of code without blank lines. However, each language implements its own editing mode and could interpret editing units differently. Function definition syntax varies across languages, so mark-defun has to be implemented differently in each language mode.

Balanced expressions could have a different meanings in different contexts, but they’re fairly consistent. Content between matching delimiters—quotation marks, parentheses, square brackets, curly braces, etc.—is generally considered a balanced expression.

Here’s where expand-region comes in. It’s typically bound to C-=. It can be used as a substitute for mark-word and mark-sexp. And if you use it repeatedly, it can replace mark-defun.

Each time you call expand-region it takes in more context. For example, suppose you’re in text mode with your cursor is in the middle of a word. The first call to expand-region selects to the end of the word. The second call selects the whole word, i.e. expanding backward to the beginning. The next call selects the enclosing sentence and the next call the enclosing paragraph.

The expand-region function works analogously when editing source code. Suppose you’re editing the bit of Emacs Lisp below and have your cursor on the slash between eshell and pwd.

(setq eshell-prompt-function
  (lambda nil
    (concat
     (eshell/pwd)
     " $ ")))

Here’s what sequential invocations of expand-region will select.

  1. /pwd
  2. /pwd/)
  3. (eshell/pwd)
  4. (eshell/pwd) " $ ")
  5. (concat (eshell/pwd) " $ ")
  6. (concat (eshell/pwd) " $ "))
  7. (lambda nil (concat (eshell/pwd) " $ "))
  8. (lambda nil (concat (eshell/pwd) " $ ")))
  9. (setq eshell-prompt-function (lambda nil (concat (eshell/pwd) " $ ")))

This is kinda tedious in this particular context because there are a lot of delimiters in a small region. In less dense code you’ll select larger blocks of code with each invocation of expand-region. Since each invocation requires only a single key (i.e. hold down Control and repeatedly type =) it’s easy to call expand-region over and over until you select the region you’d like.

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3 thoughts on “Selecting things in Emacs

  1. Thanks for all your blog posts, there are always interesting.

    Concerning selections with Emacs, I would like to mention the useful “C-w” trick after C-s (for searching):

    C-s C-w C-w … (C-w as many times you need: it selects the next word).

    Once that your selection is ok you can use C-s to start search, M-w to copy the selected text, or M-% to search/replace with the first part initialized with your selection.

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