One option could be matching 1 or more uppercase characters asserting what is directly to the right is not a lowercase character, or get the position where what is on the left is a char a-z or digit, and on the right is an uppercase char.

The use split and use a capture group for the pattern to keep it in the result.

([A-Z]+(?![a-z]))|(?<=[da-z])(?=[A-Z])

• ( Capture group 1 (To be kept using split)
  • [A-Z]+(?![a-z]) Match 1+ uppercase chars asserting what is directly to the right is a-z
• ) Close group 1
• | Or
• (?<=[da-z])(?=[A-Z]) Get the postion where what is directly to left is either a-z or a digit and what is directly to the right is A-Z

Regex demo

const pattern = /([A-Z]+(?![a-z]))|(?<=[da-z])(?=[A-Z])/;
[
  "ERISACheckL",
  "ERISA404cCheckL",
  "F401kC",
  "DisclosureG",
  "SafeHarborE"
].forEach(s => console.log(s.split(pattern).filter(Boolean).join(" ")))

Another option is to use an alternation | matching the different parts:

[A-Z]+(?![a-z])|[A-Z][a-z]*|d+[a-z]+

• [A-Z]+(?![a-z]) Match 1+ uppercase chars asserting what is directly to the right is a-z
• | Or
• [A-Z][a-z]* Match A-Z optionally followed by a-z to also match single uppercase chars
• | Or
• d+[a-z]+ match 1+ digits and 1+ chars a-z

Regex demo

const pattern = /[A-Z]+(?![a-z])|[A-Z][a-z]*|d+[a-z]+/g;
[
  "ERISACheckL",
  "ERISA404cCheckL",
  "F401kC",
  "DisclosureG",
  "SafeHarborE"
].forEach(s => console.log(s.match(pattern).join(" ")))

I have array of values which I have to separate by their uppercase. But there are some cases where the value of the array has 2, 3 or 4 serial uppercases that I must not separate. Here are some values:

ERISACheckL 
ERISA404cCheckL 
F401kC  
DisclosureG 
SafeHarborE

To be clear result must be:

ERISA Check L   
ERISA 404c Check L  
F 401k C    
Disclosure G    
Safe Harbor E

I tried using:

value.match(/[A-Z].*[A-Z]/g).join(" ")

But of couse it is not working for serial letters.