Feature: Allow string slicing and regular expressions

[christos-h]

Hey! Thank you for your work on dynfmt. It works great and is very useful.

The existing functionality is great, but for my use case it would be neat to have:

1. String slices
2. Regular expressions

To give an example:

• string slicing: {name[2..]} or {mac_addr[8..12]}
• regex extraction: {ip4cidr@^[0-9]\.[0-9]\.[0-9].\[0-9]$@{2}} to extract the third octet

Is there any plan to implement similar functionality in the future?