Sorting Text by Line, Field and Regexp in Emacs

Sorting text is such a common operation that Emacs has several commands dedicated to it, ranging from line-based sorting to complex field sorting by regexp.

Important Points

Case Sensitivity

By default Emacs will distinguish between upper and lowercase alphabet when determining sort order, and this behavior is governed by the variable sort-fold-case. Set it to t to force Emacs to ignore case differences when sorting.

Sorting Order

Emacs will by default use lexicographic sorting for all but the sort-numeric-fields command. Make sure you use the right command for the job if you want to sort numbers.

You can reverse the order of some sort commands by using a negative argument; for the commands where this does not work you must use M-x reverse-region.

Sorting by line

The simplest sorting routine is sort-lines and that function does pretty much what you would expect it to.

Let’s sort these names by line

Jerry
Elaine
George
Cosmo

And this is what the expected output should be:

Cosmo
Elaine
George
Jerry

Sorting by Paragraphs and Pages

To sort by paragraph you use the sort-paragraph command. The definition of a paragraph varies by mode, but it is usually defined as anything that is separated by one or more newlines. The variables paragraph-start and paragraph-separate control how paragraphs work.

Emacs will treat something as a page if it is delimited by the form feed character, which is ASCII 12. To sort by page use the sort-pages command.

Sorting by Fields

Sorting by a field is much akin to sorting tabulated data: you have a list of data and you wish to sort by only a subset of that data — a field.

Emacs has two commands to do this: sort-fields for most things; and sort-numeric-fields for numeric sort order. Both require a numeric argument if you want to sort by anything other than the first field, where a field is defined as anything separated by a whitespace such TAB or SPACE. If you pass a negative argument, then Emacs will count backwards when picking the field to use.

I recommend that you use the numeric sort if you intend to sort by numbers as Emacs is clever enough to detect hexadecimal (if beginning with 0x) and octal (if beginning with 0) or an entirely different base, as determined by the sort-numeric-base variable, which defaults to 10 (for decimal.)

If you don’t sort numbers using the numeric command you risk sorting your numbers the wrong way.

Sorting fields

Let’s sort by first name and then by last name.

Jerry Seinfeld
Cosmo Kramer
Elaine Benes
George Costanza

To sort by first name I type M-x sort-fields. No need for a numeric argument as it will default to one, which is the first field — the first name field.

Cosmo Kramer
Elaine Benes
George Costanza
Jerry Seinfeld

OK, so that’s nice and easy. Sorting by last name is just as easy. Type M-2 M-x sort-fields and you should see this:

Elaine Benes
George Costanza
Cosmo Kramer
Jerry Seinfeld

Sorting fields numerically

Sorting numerically with sort-numeric-fields is much the same as with sort-fields, though I will highlight why it is important to use the correct command when you want to sort numbers.

Consider the following data.

4 – Locke
8 – Reyes
15 – Ford
16 – Jarrah
23 – Shephard
42 – Kwon

I want to sort by the number, but I will do so with sort-fields and not sort-numeric-fields.

15 – Ford
16 – Jarrah
23 – Shephard
4 – Locke
42 – Kwon
8 – Reyes

Hmm. Not exactly the intended output. The sort-fields command (indeed, so would sort-lines) will sort lexicographically and not numerically.

Again, with sort-numeric-fields this time.

4 – Locke
8 – Reyes
15 – Ford
16 – Jarrah
23 – Shephard
42 – Kwon

Much better.

Sorting by Regular Expression

I love this command. It’s very powerful and lets you do sorting with the precision of a regular expression.

The sort-regexp-fields works by searching the region for everything that matches a record regexp and for each match it finds, it looks in that record for a key regexp. The key is used to determine how to sort each record.

What this means, in practical terms, for you is that you can sort just a subset of your text and leave the rest untouched. In other words, you could, if you wanted to, sort only parts of the text but leave the rest as it were; for example, sort everybody’s first name but without shuffling the last name as well.

The key prompt, if left blank, will default to \&, which is the entire match string. If you have capturing groups in the record regexp, you can use the usual \N subexpression matching.

Emulating sort-lines

To emulate sort-lines you can run sort-regexp-fields with these parameters:

Regexp specifying records to sort: ^.*$
Regexp specifying key within record: \&

Complex sorting

Say you want to sort the text below by the last character in each last name.

Cosmo Kramer
Elaine Benes
George Costanza
Jerry Seinfeld

Invoke sort-regexp-fields and use the following parameters:

Regexp specifying records to sort: \w+\(\w\)$
Regexp specifying key within record: \1

The resultant output is what you would expect — almost!

Cosmo Costanza
Elaine Seinfeld
George Kramer
Jerry Benes

The sort command only sorted the last name — which is all the record regexp matched — and left the first names alone. Let’s try again with a revised record parameter:

Regexp specifying records to sort: ^.+\w+\(\w\)$
Regexp specifying key within record: \1

Now the output is correct:

George Costanza
Jerry Seinfeld
Cosmo Kramer
Elaine Benes

So regexp sorting is really powerful but can introduce subtle errors you may not spot right away. Always match the entirety of each unit — each record — and never do partial matchess unless that is what you want, of course. Use the subexpression matches to pick out the actual keys you want to sort by.

Conclusion

Sorting in Emacs is really powerful and a very useful tool if you do any sort of data scrubbing or manipulation. But beware the differences between lexicographic and numeric sorting when you work with numbers, and double-check the regexps you use when you sort by regexp fields.