bug: Not compliant with full list of characters allowed in cite key

[shyamd]

There's more characters than what the current regex allows in the cite key
https://tex.stackexchange.com/questions/408530/what-characters-are-allowed-to-use-as-delimiters-for-bibtex-keys

[filaretov]

Would you be open to contributions to address this?

I have played around with a solution on this branch.

The commit message contains a description of the process used to determine the proposed regex pattern.