VS Code: How to convert snippet placeholder from camelCase to SCREAMING_SNAKE_CASE?

Question

I'd like to create a VS Code snippet for creating redux reducers.

I would like to have a snippet with placeholder that expects camelCase and then transform a matching placeholder to SCREAMING_SNAKE_CASE.

Here's my attempted snippet, which is not working:

"test": {
    "prefix": "test",
    "body": "${1} -> ${1/([a-zA-Z])(?=[A-Z])/${1:/upcase}_/g}"
},

Which produces a non-desired result:

changeNetworkStatus -> changE_NetworK_Status

Desired Flow

1. type test (name of snippet)
2. hit tab to load the snippet.

3. type changeNetworkStatus to result in:

   changeNetworkStatus -> changeNetworkStatus
   

4. hit tab to get expected result of:

   changeNetworkStatus -> CHANGE_NETWORK_STATUS
   

How can I change my snippet code to get the desired result?

Here's a related solution which requires a different flow.

Answer 1

"camelCaseModify": {
    "prefix": "test",       
    "body": [

       //  first inefficient try, works for up to three words
       //  "${1} -> ${1/^([a-z]*)([A-Z])([a-z]+)*([A-Z])*([a-z]+)*/${1:/upcase}_$2${3:/upcase}${4:+_}$4${5:/upcase}/g}"

       "${1} -> ${1/([a-z]*)(([A-Z])+([a-z]+))?/${1:/upcase}${2:+_}$3${4:/upcase}/g}"           
    ],
    "description": "underscore separators"
},

This works with any number of camelCase words, from one to infinity...

The ${2:+_} means "if there is a capture group 2 then append an underscore." If there isn't a second word/capture group then groups 3 and 4 will be empty anyway because they are within capture group 2. Capture Group 2 is always the next Word (that starts with one capital and followed by at least one small letter).

for example, using changeNetworkStatus:

Match 1

Full match    0-13    `changeNetwork`
Group 1.      0-6     `change`
Group 2.      6-13    `Network`
Group 3.      6-7     `N`
Group 4.      7-13    `etwork`

Match 2

Full match    13-19    `Status`
Group 1.      13-13     ``
Group 2.      13-19     `Status`
Group 3.      13-14     `S`
Group 4.      14-19     `tatus`

Match 3

Full match    19-19    ``
Group 1.      19-19    ``

Sample Output:

abcd -> ABCD
twoFish -> TWO_FISH
threeFishMore -> THREE_FISH_MORE
fourFishOneMore -> FOUR_FISH_ONE_MORE
fiveFishTwoMoreFish -> FIVE_FISH_TWO_MORE_FISH
sixFishEelsSnakesDogsCatsMiceRatsClocksRocks -> SIX_FISH_EELS_SNAKES_DOGS_CATS_MICE_RATS_CLOCKS_ROCKS

Using regex101.com really helps to visualize what is going on!