How can I subscript single letter words and single letter words only?

[RobberFox]

For example I want be able to have x1 -> x_{1} or A1 -> A_{1} etc. without having sin3 -> sin_{3} or \dots1 -> \dots_{1} etc. I would like to achieve this without changing my every snippet and adding a white space to it like this "something-something" $0
I have this snippet:
{trigger: /(\b[A-Za-z]\b)(\d)/, replacement: "[[0]]_{[[1]]}", options: "rmA", description: "Auto letter subscript", priority: -1},
The idea is to match any single letter word and single letter word only, but this doesn't work - what am I doing wrong?

[RobberFox]

Sorry for my my rushed question. I resolved this issue in 5 minutes by removing the \b in the end (because I need a whitespace before the letter and not also after it)