I cant understand ?= expression right maybe?

Tell us what’s happening:
My question here is simple.It seems that i didn’t get something right on the regex lesson.Why here i solved the challenge using in the split method this /(?=[A-Z])|_|\s/ and i couldn’t solved with this /(?=[A-Z])|(?=_)|(?=\s)/.


Your code so far

function spinalCase(str) {
return str;

spinalCase('This Is Spinal Tap');
spinalCase("AllThe-small Things")
spinalCase("Teletubbies say Eh-oh");

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Challenge: Spinal Tap Case

Link to the challenge: