piapro.jp is a Japanese website for user-uploaded content, closely related to Vocaloid and piapro characters such as Hatsune Miku. Anyone can make an account, upload content, leave comments and follow other users.

While users can upload content in categories such as illustrations and 3D models, the categories relevant to MusicBrains are music and text (for lyrics).

Uploaded music can be streamed for free without an account. With an account, mp3 files with original metadata can be downloaded. In addition to music, users can upload vocal synthesiser data (vsq, vsqx) used for the song (not relevant to MusicBrains). Some music entries (example) have an image associated with it, serving as the album art. The original full resolution image can be downloaded with an account by going to the relevant entry in the images category.

The links are very stable, here's an example from twitter in 2011 that still works (uses http://).

Here are some example links and the appropriate relationships to MusicBrains entities:

• https://piapro.jp/t/rVAV
  • Recording / Download for free this example has the original and instrumental versions of the recording under the same piapro entry
  • Recording / Free streaming only the original version of the recording can be streamed here (the instrumental is download-only)
  • Release / Download for free the two tracks available in the download make up a complete single

• https://piapro.jp/t/A3gc
  • Work / Lyrics

• https://piapro.jp/matt9five
  • Artist / Social network

As you can see, one cannot differentiate between the categories (music, text, illustration) based on the URL alone, so all links need to be available for all entity types.

Looking at the IDs, they are comprised of numbers, uppercase and lowercase letters, hyphens and underscores:

^https?:\/\/piapro\.jp\/t\/([0-9a-zA-Z_-]{4})$

User accounts seem to follow the same restrictions (with possibly the hyphen being invalid, unsure) and are case-insensitive. But there is a canonical casing.
^https?:\/\/piapro\.jp\/([0-9a-zA-Z_-]+)$

User accounts are differentiated from regular website pages with a trailing slash. Compare https://piapro.jp/music (user account) and https://piapro.jp/music/ (music listing page).

The attached screenshot shows the download modal of https://piapro.jp/t/rVAV - two distinct mp3 files.