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#+TITLE: Aligning columns in Emacs
#+DATE: 2021-03-26T11:27:34-04:00
#+DRAFT: false
#+DESCRIPTION: How to align text separated by whitespace using Emacs
#+TAGS[]: emacs elisp
#+KEYWORDS[]: emacs elisp
#+SLUG:
#+SUMMARY:
#+ATTR_HTML: :alt Before and after columns aligned
#+ATTR_HTML: :title Before and after aligned columns
[[file:cover.png]]
I've been writing a lot of GraphQL resolvers in ruby at work recently,
and frequently run into a situation where I have to align many columns
of text by hand. I figured this isn't a very unique problem, and that
there was probably already some code within Emacs to handle it.
As it turns out I was correct in my assumption. There's an excellent
built-in package called =align= that takes care of just that.
Many of the features of the =align= package are mode-specific, but I
just wanted a function that can align columns regardless of which mode
they're used in. There's a function that's part of the package that
can be used for just that fortunately, =align-regexp=, which takes a
regular expression as input and uses it to guide the alignment.
I was able to use it to make a generic function.
#+begin_src elisp
(defun align-non-space (BEG END)
"Align non-space columns in region BEG END."
(interactive "r")
(align-regexp BEG END "\\(\\s-*\\)\\S-+" 1 1 t))
#+end_src
Let's walk through what's going on here.
- =BEG= and =END= are the beginning and end positions of the area to be aligned
- =(interactive "r")= tells Emacs that the function is interactive,
meaning that it can be called from the =M-x= menu
- ="r"= tells Emacs that when the function is called interactively, it expects a
region (beginning and end points) as arguments
- =align-regexp= where the work is happening. This function has the
following signature.
#+begin_src elisp
(align-regexp BEG END REGEXP &optional GROUP SPACING REPEAT)
#+end_src
- =BEG= and =END= is the region that it expects as the first arguments
- ="\\(\\s-*\\)\\S-+"= is an Emacs regular expression. Backslashes
are doubled because they need to be escaped in a string literal
- =\( \)= is a capture group
- =\s-= is a [[https://www.gnu.org/software/emacs/manual/html_node/elisp/Regexp-Backslash.html][regular expression construct]] specific to Emacs which
specifies a type of /syntax/ which is to be matched.
- =-= refers to the [[https://www.gnu.org/software/emacs/manual/html_node/elisp/Syntax-Class-Table.html][whitespace characters]]
- =*= means match zero or more
- =\S-= is similar to the previous construct, but instead means to
match anything /other/ than whitespace
- =+= means one or more
- =1= here refers to the group within the regex that will be
modified to align the fields
- =1= is the number of spaces between fields once aligned
- =t= indicates to repeat the rule multiple times on the same line
To use this function, simply highlight a region you want to align and
run =M-x align-non-space=.
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