Get Seed from wallet on regtest mode
You are looking for the dumpwallet command.     

While in regtest open the console a/k/a debug window, and type:
dumpwallet "C:\myfolder\dump.txt"

This will create a text file with all your keys. At the top you will see your extended master private key. And if you are looking for your HD seed, do a control+f for hdseed, you should find a key with the label hdseed=1.  

https://bitcoincore.org/en/doc/0.18.0/rpc/wallet/sethdseed/