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Leveraging Regexes and Git to Find/Replace Dates in Markdown Files

Recently I’ve been moving my blog off Jekyll as a static site generator. Nothing against Jekyll, I love it. I just don’t want to deal with Ruby anymore. JavaScript all the way. In moving things over, I found a duplication of data I wanted to fix. In fixing it, I thought, “I better write this down...I might need to do this again some day...” so here it is.

Jekyll requires that you structure the names of your markdown post as a combination of post date and post slug (YYYY-MM-DD-slug-of-post.md) which results in file names like this:

Inside of each post file, I had a duplication of data in that the date of the post was repeated in the YAML front-matter:

title: Why jQuery is Great
Let me tell you the answer to all your problems—jQuery...

When switching over to my new, homemade blogging-engine (more on that in a future post), I realized I didn’t need the date in two places. I only needed it in one. This actually got me wondering: are there places where the date in the file’s name and the date in the file’s front-matter don’t actually match? A quick node script helped me find out:

const fs = require("fs");
const path = require("path");

const dir = path.join(__dirname, "/src/client/posts");

fs.readdirSync(dir).forEach(file => {
  const contents = fs.readFileSync(path.join(dir, file)).toString();
  const date = /date: (.*)/g.exec(contents);
  if (date) {
    const dateinfile = date[1];
    const dateinfilename = file.slice(0, 10);
    if (dateinfile !== dateinfilename) {
      console.log(file, dateinfile, dateinfilename);
  } else {
    console.log("No `date:` found for file: ", file);

Through this little node script, I discovered four of my posts had mismatched date values. So I checked the live version of my blog (which at the time was still rendered by Jekyll) and discovered that Jekyll appeared to be using the date: in the front-matter and not the file name. So I decided to do a couple things:

  1. Rename all files with mismatching date values so the date in the file’s name matched the date in the file’s front-matter
  2. Remove all occurences of date: from post front-matter

Number one was relatively easy. My node script only surfaced four files where values mismatched. So that was an easy one to do by hand.

Screenshot of the result of my node script

Number two was a little bit tricker. After dusting off the part of my brain that deals with regexes, I used the find/replace in VSCode to essentially look through all my posts/*.md files and remove any lines that started with date:.

My first attempt found me using this regex pattern: ^date:.*$. That seems right, right? Start at the beginning of the line (^) look for date: and then anything following that (.*) until the end of the line ($). VSCode made seeing the results of my search quite easy.

Screenshot of the result of my first regex search in VSCode

After doing the find/replace, I looked at my file changes and realized I wasn’t capturing the entire line. The computer had found all my date: strings and removed them, leaving me with an empty line in each post file:

Screenshot of my first regex changes in git

As you can see, I now had an empty line in each file. There was still dust on the regex part of my brain, so I read some more about the $ character on regex101 and found the following (emphasis mine):

$ Matches the end of a string without consuming any characters. If multiline mode is used, this will also match immediately before a newline character.

Ah, so the new line character wasn’t being captured. Reverting my changes was easy with git, (seeing as I had no other changes in my working directory) all I had to type was git checkout -- . and I was back to having date: in all my post files.

I then changed my regex to capture the new line character (\n) and performed another search/replace (^date:.*$\n):

Screenshot of the result of my second regex search in VSCode

Ahh, sweet, sweet success!

Screenshot of my second regex changes in git

As you can see, that gave me precisely what I wanted: remove every line that started with date:. Working with git made this really easy. I could do search/replaces all day, ensure I got what I wanted by previewing the changes in git, then either committing or reverting my changes.

You might be reading this post and thinking “pff, this isn’t really anything. I do stuff like this all day every day!” And I get it. This is “just a simple regex find/replace”. But this, to me, is what makes computering so much fun. For me, there’s a big sense of satisfaction when you leverage what tools you know to do a task that you know would’ve otherwise taken you an incredible amount of time to do by hand.

That’s all. The end.