[RFC17] add regex to quickly recognize if a comment fits this format

rfc/0017_transaction_comment_encryption.md

@@ -23,7 +23,7 @@ Use NaCl function `crypto_sign_ed25519_pk_to_curve25519`.
 
 Let `Pr' = crypto_sign_ed25519_pk_to_curve25519(Pr)`
 
-## Generate symetric encryption key
+## Generate symmetric encryption key
 
 We use nacl function [crypto_box_beforenm]. Then use scrypt with a random nonce.
 
@@ -46,7 +46,6 @@ With following parameters for scrypt:
 |**p** | 1 |
 |**dkLen** | message length |
 
-
 ## Encrypt with XOR cipher
 
 Encrypt bit per bit with XOR cipher.
@@ -112,6 +111,10 @@ The DUBP protocol allows up to 255 characters in the comment field, so it is pos
 
 There are at least 21 bytes of metadata, so the maximum size of a message is **170 bytes**.
 
+## Quickly recognize if a comment fits this format
+
+You can quickly recognize if a comment fits this format with the following regex: `J7YB[0-9a-zA-Z+/]{24,}`
+
 ## Encrypt transaction comment (Issuer side)
 
 Let `m` the user message bytes encoded in UTF8.