Regex-transformation with text and numbers

I am having troubles making my regex-statement to work as intended.

Regex statement: ([a-z][a-z\d]+)(?=([A-Z][a-z\d]+))

Starting string: regex2Hard4Me

current result: REGEX2_HARD4_ME

desirable result: REGEX_2_HARD_4_ME

Any help is highly appreciated.

>Solution :

You can match a single char a-z or digit, and assert a char A-Z or digit to the right:

[a-z0-9](?=[A-Z0-9])

In the replacement use the full match followed by an underscore.

Regex demo

Output

regex_2_Hard_4_Me

Then uppercase the string to get REGEX_2_HARD_4_ME