[AlphaCommerceHub] Which characters are considered «special» and «not special» for the AlphaHPP's `MerchantTxnID` parameter?

Extensions AlphaCommerceHub
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The description of the MerchantTxnID parameter in the «AlphaHPP» → «Paypage Request Reference» → «Request a Paypage Session» documentation section says:

Supported special characters are / and .

But which characters are considered «special» and «not special» exactly?

For example, I have noticed that AlphaHPP accepts the - (dash) character.
Is it «special» or «not special», and why exactly?

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See also:

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Guide is being updated, following characters are considered special and are supported in this field (enclosed in quotes)

#.: {}\-/

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Implemented in 1.0.3.
https://github.com/mage2pro/alphacommercehub/issues/90