How to convert snippet placeholder from CamelCase to snake_case - regex

Question

I would like to create a VS Code snippet where I input a part in CamelCase, and the same string is output in snake_case at some other place in the snippet.
Based on this SO post Here's my attempted snippet, but I have a trailing _ that needs to be removed by hand, not ideal:
"test": {
"prefix": "test",
"body": "${1} -> ${1/([A-Z])+([a-z]+)/${1:/downcase}${2}_/g}"
},

Answer 1

"camelToSnakeCase": {
"prefix": "test",
"body": [
"${1} -> ${1/([A-Z][a-z]+$)|([A-Z][a-z]+)/${1:/downcase}${2:/downcase}${2:+_}/g}"
],
"description": "transform from CamelCase to snake_case"
}
In order to differentiate between some last capture group like Abcd and the preceding capture groups, I used an alternation:
([A-Z][a-z]+$)|([A-Z][a-z]+) must be in this order
so group 1 will be at the end of the input because of the $ indicator and group 2 will not be at the end. And group 2s will always have at least one more group after them. Then, using the conditional ${2:+_} only insert an underscore if there is a group 2 - because there must be a following group 1.
This keybinding version also works if you have a different workflow:
{
"key": "alt+3", // whatever keybinding you wish
"command": "extension.multiCommand.execute",
"args": {
"sequence": [
"cursorWordLeftSelect",
"editor.action.transformToSnakecase",
"editor.action.transformToLowercase",
// "cursorLineEnd" // if you want this
]
},
"when": "editorTextFocus && !editorHasSelection"
}
Type your word and trigger the keybinding. Uses the macro extension multi-command.