Find, convert and replace dates with Vim substitutions

Vim’s substitution command is a powerful way to make changes to text files. Besides finding and replacing text using regular expressions, substitutions can call out to external programs for more complicated replacements. By using the date utility from a substitution, Vim can convert all dates in a file to a different format and replace them all at once.

Finding, converting and replacing dates with Vim substitutions

The input file is an HTML page with a list of articles. Each article includes a <time> tag with a value and a datetime attribute to show the publication date.

The input file

<ul>
  <li>
    <article>
      <time datetime="2017-10-17">2017-10-17</time>
      <span><a href="/vim-reformat-dates/">Find, convert and replace dates with Vim substitutions</a></span>
    </article>
  </li>
  <li>
    <article>
      <time datetime="2017-09-19">2017-09-19</time>
      <span><a href="/open-source-maintainability/">Keeping open source software projects maintainable</a></span>
    </article>
  </li>
  <li>
    <article>
      <time datetime="2017-08-22">2017-08-22</time>
      <span><a href="/mix-proper/">Property-based testing in Elixir using PropEr</a></span>
    </article>
  </li>
  …

We need to convert the dates’ values to a friendlier format that includes the full month name (“September 19, 2017”), while keeping the datetime attributes in their current format.

The result: articles with reformatted dates

<ul>
  <li>
    <article>
      <time datetime="2017-10-17">October 17, 2017</time>
      <span><a href="/vim-reformat-dates/">Find, convert and replace dates with Vim substitutions</a></span>
    </article>
  </li>
  <li>
    <article>
      <time datetime="2017-09-19">September 19, 2017</time>
      <span><a href="/open-source-maintainability/">Keeping open source software projects maintainable</a></span>
    </article>
  </li>
  <li>
    <article>
      <time datetime="2017-08-22">August 22, 2017</time>
      <span><a href="/mix-proper/">Property-based testing in Elixir using PropEr</a></span>
    </article>
  </li>
  …

The input file has more than forty articles, so replacing them all by hand would be a lot of error-prone work. Instead, we write a substitution that finds all dates in the file and replaces them with a reformatted value.

Finding the dates

The first step in replacing the dates is finding where they are in the input file and making sure not to match the ones in the datetime attributes.

To find all dates in the file, we could use ....-..-.. (esc /....-..-..) as our search pattern to match the date format. However, this pattern’s results will include all matching dates in the file, including the ones in the <time> tags’ datetime attributes.

<time datetime="2017-09-19">2017-09-19</time>

In the input file, all <time> values are immediately followed by the less-than sign from the closing </time> tag. To prevent the datetimes from the attributes to be included in the results, we could include the less-than sign in the search pattern and make sure to add it back when replacing.

....-..-..<

Hoever, Vim supports setting the start and end of the match in the search pattern using the \zs and \ze pattern atoms. By prefixing the < in our search pattern with \ze, the pattern finds all dates followed by a less-than sign, but doesn’t include it in the match, meaning it won’t be replaced.

....-..-..\ze<

Reformatting dates from the command line

We need the month’s full name in the date replacements, so we can’t reorder the input value (“2017-09-19”) to get the result we want. Instead, we need to call out to an external utility that knows month names and can convert between date formats.

The `date` utility

The date utility returns the current date and time. The desired output format is passed in through a trailing argument that starts with a plus sign. It needs to be wrapped in quotes if it contains any whitespace.

$ date
Thu Oct 17 09:41:10 CEST 2017
$ date +%Y-%m-%d
2017-10-17
$ date +"%B %d, %Y"
October 17, 2017

On macOS (or other derivatives of BSD) passing a date to the date utility overwrites the system date unless the -j flag is passed. With that flag, the utility allows passing a test date to be reformatted. The test date needs to be formatted as [[[mm]dd]HH]MM[[cc]yy][.ss] by default or we can pass a custom input format with -f.

$ date -j110200361991.35
Sat Nov  2 00:36:35 CET 1991
$ date -jf %Y-%m-%d 1991-11-02
Sat Nov  2 09:41:27 CET 1991

GNU date uses the --date argument to pass in dates, and it figures out the input format on its own.

# date --date=1991-11-02
Sat Nov  2 00:00:00 UTC 1991

By setting the input and output format, we can use the date utility to reformat dates.

$ date -jf %Y-%m-%d 1991-11-02 +"%B %d, %Y" # BSD
November 02, 1991

# date --date=1991-11-02 +"%B %d, %Y" # GNU
November 02, 1991

We reformat each match of our search pattern to our desired format (“September 19, 2017”) with the date utility. We use "%Y-%m-%d" as the input format to match the results from the search pattern. The output format is "%B %d, %Y" to produce the month’s full name, the date’s number, a comma and the year number.

With these formats the date utility reformats 1991-11-02 to November 02, 1991.

$ date -jf "%Y-%m-%d" "1991-11-02" +"%B %d, %Y"
November 02 1991

-j: Don’t try to set the system date
-f "%Y-%m-%d": Use the passed input format instead of the default. In this case "%Y-%m-%d" to match the input format (1991-11-02).
"1991-11-02": An example date to be parsed using the input format passed to -f.
+"%B %d, %Y": The output format, which produces November 02, 1991.

Calling out to external utilities from substitutions

We know how to find all dates in the file, and how to convert a date to another format from the command line. To replace all found dates with a reformatted version from the date utility, we need to run an expression from a substitution.

Using the search pattern we prepared earlier, we can find and replace all date values from the input file with a substitution. For example, we could overwrite all dates with a hardcoded value:

:%s/....-..-..\ze</November 2, 1991/gc

....-..-..: The search pattern to find all dates in the file.
November 2, 1991: The literal substitute string to replace the dates with a hardcoded one.

Instead of inserting a hardcoded substitute string, we need to run an expression for each match to get its replacement.

Expressions

When a substitute string starts with \=, Vim evaluates it as an expression.

We can call Vim’s built in functions from an expression. To replace all numbers in a file with the number of the line they’re on, we use the line() function from an expression in the substitute string.

%s/\d\+/\=line('.')/gc

\d\+: The pattern to match all numbers (\d) in the file. Multi-digit numbers (42, 785, 14281) are matched as one number by using \+.
\=line('.'): The substitute string with an expression (\=) to call the line() function. Passing '.' as the function’s argument returns the current cursor position, which is used to replace the match.

Vim provides the system() function to call out to an external command and use the result as the replacement string. To replace all numbers in a file with a random number, we call echo -n $RANDOM with the system() function.

:%s/\d\+/\=system('echo -n $RANDOM')/gc

We use the system() function from an expression (\=) to call out to the date utility. Sticking with hardcoded dates for now, we can use the utility to convert a date’s format from “1991-11-02” to “November 2, 1991” before inserting it into the file:

:%s/....-..-..\ze</\=system('date -jf "%Y-%m-%d" "1991-11-02" +"%B %d, %Y"')/gc

....-..-..\ze<: The search pattern to find all dates in the file.
\=system('date …'): An expression that uses the system() function to execute an external command and returns its value as the substitute string.
'date -jf "%Y-%m-%d" "1991-11-02" +"%B %d, %Y"': The date command as a string, with a hardcoded date ("1991-11-02") as its input date argument. This date matches the format of the search pattern’s matches.

Warning:

This substitution produces a newline before the <time> tag, because the date utility appends one to its output. We’ll remove these later while discussing nested substitutions.

The replacement value is still hardcoded (“1991-11-02”), so this substitution will overwrite all date values in the file to a date in 1991. To put the matched date values back in the file, we need to pass them to the date command.

To pass the matched date to the call to date in our expression, we need to break out of the string passed to the system() function and replace the hardcoded date with a call to submatch(0) to insert the whole match.

:%s/....-..-..\ze</\=system('date -jf "%Y-%m-%d" "'.submatch(0).'" +"%B %d, %Y"')/gc

Running this substitution will turn all <time> tags from the input file to our desired format, but it puts a newline before the closing </time> tag.

The current result, with an added newline before the closing </time> tag

<article>
  <h1>Keeping open source projects maintainable</h1>
  <time datetime="2017-09-19">September 19, 2017
</time>
</article>

...

Nested substitutions

A newline is appended to the result of the date command, which ends up in the file after running the substitution. Since there’s no way to get the date command to omit the newline, we need to take it out ourselves.

We can run a second substitution to remove them after running the first one:

:%s/\n<\/time>/<\/time>/g

To keep the original substitution from adding newlines in the first place, we can pipe the result from the date command to tr to remove the newline with the -d argument:

:%s/....-..-..\ze</\=system('date -jf "%Y-%m-%d" "'.submatch(0).'" +"%B %d, %Y" | tr -d "\n"')/gc

Another option is to take the newline out with a nested substitution.

The `substitute()` function

Vim’s substitute() function replaces strings and can be run from an expression in a substitution. Nested substitutions are useful for transforming the result of another function.

The function (substitute()) works like the substitute command (:s), and takes the same arguments, so substitute("input", "find", "replace", "g") is equivalent to running :%s/find/replace/g in a file.

The substitute() function works like the substitute command (:s[ubstitute]) in Vim’s command line and takes the same arguments. The first argument is the input, then the search pattern, the substitute string, followed by optional options. substitute("input", "find", "replace", "g") is equivalent to running :%s/find/replace/g in a file.

:echom substitute("October 02, 1991", "October", "November", "")

"October 02, 1991": The input string to run the substitution on.
"October": The search pattern.
"November": The substitute string.
"": Options, like in a “regular” substitution. This example doesn’t use the g option because we’re sure there’s only one match in the input string, so it isn’t necessary.

If an external command called from a substitution returns a trailing newline (like echo would without the -n flag), we can use the substitute() function to take it out before the match is replaced.

:%s/October/\=substitute(system('echo "November"'), "\n", "", "")/gc

We wrap the call to the date utility in a nested substitution using the substitute() function. It takes the result, matches the newline ("\n") and replaces it with an empty string.

:%s/....-..-..\ze</\=substitute(system('date -jf "%Y-%m-%d" "'.submatch(0).'" +"%B %d, %Y"'), "\n", "", "")/gc

Now our substitution will turn the <time> tags into the correct format, without adding that extra newline.

The result

<ul>
  <li>
    <article>
      <time datetime="2017-10-17">October 17, 2017</time>
      <span><a href="/vim-reformat-dates/">Find, convert and replace dates with Vim substitutions</a></span>
    </article>
  </li>
  <li>
    <article>
      <time datetime="2017-09-19">September 19, 2017</time>
      <span><a href="/open-source-maintainability/">Keeping open source software projects maintainable</a></span>
    </article>
  </li>
  <li>
    <article>
      <time datetime="2017-08-22">August 22, 2017</time>
      <span><a href="/mix-proper/">Property-based testing in Elixir using PropEr</a></span>
    </article>
  </li>
  …

Thanks Wouter Vos and Rico Sta. Cruz for feedback on the substitution command styling, u/Vurpius for suggesting \ze and Ben Sinclair for suggesting piping through tr.