Insertion of the leading zeros

Hello.
I’ve got a little misunderstanding. There is a text file with such kind of list:
131_1.txt different symbols
131_2.txt different symbols
131_3.txt different symbols
…
131_10.txt different symbols
…
131_100.txt different symbols
…
131_1000.txt different symbolsI have to insert leading zeros after prefics to get this kind of the list:
131_0001.txt different symbols
131_0002.txt different symbols
131_0003.txt different symbols
…
131_0010.txt different symbols
…
131_0100.txt different symbols
…
131_1000.txt different symbolsIs there an opportunity to perform it using Notepad++? I can find string “131_1.txt” with regular expressions, but I don’t know how to replace or just insert zeroes.
I can do it manually with vertical blocks, of course, but it’s interesting to do it automatically ) 
This thread will have information to help you in this task: https://notepadplusplus.org/community/topic/13988/findreplaceissues
Since you have experience with regular expressions I will not do it for you, but will rather leave you to it! :)

Brilliantly, it works! Thank you so much)