How to write a RegEx that would capture a piece of a string, unless the string ends with a certain pattern?

I’ve started studying RegexOne, and there’s a exercise in which we must capture a piece of a string until the "." , as long as the string ends with .pdf

match   file_a_record_file.pdf
match   file_yesterday.pdf
skip    testfile_fake.pdf.tmp   

But I wanted to get a bit deeper and capture this piece of a string, unless the string contains more than 3 characters after a dot character "."

I tried using

^(\w+(?!([.](.{4,}))$))

but of course it didn’t work. How could I correct this pattern, considering the JavaScript RegEx library? (I don’t want a function, only a pattern, if that’s possible).
I guess it would be more flexible if I could avoid using $, but I’d accept any answer matching the question. Thank you all in advance.

>Solution :

I belive this is what you are trying to do:

mySting.match(/(.*)\.pdf$/)

Match   file_a_record_file.pdf
Match   file_yesterday.pdf
null    testfile_fake.pdf.tmp

Edit: the string without the extension is stored on mySting.match(/(.*)\.pdf$/)[1]

For the 3 characters match after the first dot

myString.match(/^([^\.]+)\.[\w]{1,3}$/);

Explanation: match everything that is not a dot + match the first dot + match a word that contains 1 to 3 characters at the end of string