---
title: "Semver: The unknown buildMetadata"
tags: ["npm", "versioning"]
createdAt: 2022-07-14
lastUpdated: 2022-07-14
description: "The semver’s spec mentions a group called “buildMetadata”. If you’ve never heard of this before, you’re not alone"
atUri: "at://did:plc:wifmfjqoxnruni5h62e6zp2y/site.standard.document/3mn6dw4jtrf2w"
---

I was reading [semver’s spec](https://semver.org/) and I noticed something weird in the regex:

![Screenshot of semver spec saying "And one with numbered capture groups instead (so cg1 = major, cg2 = minor, cg3 = patch, cg4 = prerelease and cg5 = buildmetadata)"](@assets/dyn2hb56xwjex3cyd6hr.png)

This is the 1st time that I ever encountered the mention of `buildmetadata`.

### Let's deconstruct that a bit

When you read the regex, it mentions at the beginning the required `major`, `minor`, and `patch`: a valid semver version needs to be at least `MAJOR.MINOR.PATCH`, for instance `1.2.3`.

Then the version can contain an option `prerelease` tag starting with a **`-`**, for instance `1.2.3-hello` and this prelease tag can be anything made of alpha numerics (0-9 or a-z or A-Z), `.` and `-`.

But what surprised me is the last one: the `buildmetadata` prefixed with a **`+`** (same thing as the prerelease so any alpha numerics, or `.`, or `-`).
So for instance `1.2.3+build` is valid, and so is `1.2.3-beta.1+build.linux`

### Verification

Just to be sure, I run this in runkit, and indeed it can properly parse `1.2.3-pre.release.4+build.meta.5.6` as:

```json5
{
  major: 1,
  minor: 2,
  patch: 3,
  prerelease: ["pre", "release", 4], // here 4 is even a number
  build: ["build", "meta", "5", "6"], // but here 5 and 6 are strings
  version: "1.2.3-pre.release.4",
  raw: "1.2.3-pre.release.4+build.meta.5.6",
}
```

See for yourselves:

![semver properly parses `1.2.3-pre.release.4.5+build.meta.6.7`](@assets/niejqzhlppdlxyaj1tez.png)

### Discussion

I've seen plenty of times the prerelease being used (and I used it myself a few times too).

For instance, React uses it a lot:

![Sample of React versions on NPM with a lot of prereleases](@assets/p00o9cthqgf0s4dlhymu.png)

My topic for a conversation with you is: Have you ever seen this used anywhere on NPM (or else where)? And if so, for what purpose?