[OskarS]

And slashes as well, which is a magic character in both urls and file systems. Means you can't reliably use normal base64 for filenames, for instance. That might seem like a niche use-case, but it's really not, because you can use it for content-based addressing. Git does this, names all the blobs in the .git folder after their hash, but you can't encode the hash with regular base64.

[layer8]

There’s the URL- and filename-safe variant of Base64 [0]. Decoders can support it simultaneously and transparently.

[0] https://www.rfc-editor.org/rfc/rfc4648.html#section-5

[cyanydeez]

you can also manually replace the with urlsafe codes