Like I said, the notation for string slicing is so trivial to learn and use that it’s unlikely anyone will take the time to build a dedicated tool for it. To be clear, it’s not even “building a template” but merely using python’s native string-slicing capabilities.
In contrast, regex is vastly more complicated. Composing a proper regex pattern can be far more challenging than slicing notation, which is merely [start:stop], and that’s why there are numerous books on the subject and online regex tools.
Anyway, don’t forget to vote for your own FR.