possible characters base64 url safe function

php base64

Base64 encoded strings may contain the characters a-z A-Z 0-9 + / =.

You're removing the right-padding = and replacing + with - and / with _.

So in your case, the possible characters are a-z A-Z 0-9 - _

php base64

The range of possible characters returned are:

In your regex-style, that would be [a-zA-Z0-9_-].

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