camelCase to snake_case
Read a variable name written in camelCase and print it in snake_case: lower case, with an underscore before each word that started with a capital.
userFirstName becomes user_first_name.
Input
userFirstName
Expected output
user_first_name
Input
total
Expected output
total
Show a hint
A regular expression /[A-Z]/g finds every capital letter; replace each with an underscore plus its lower-case form.
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