Match number between characters

Question

Here is some examples 158811_23.jpg 151_188.gif

How to match the number between _ and .jpg?

Answer 1

If you want to match the numbers between _ and .jpg but exclude the _ and .jpg then use:

(?<=\_)[0-9]+(?=\.jpg)

(?<=\\_) is called a lookbehind. Anything after the _ except the _

(?=\\.jpg) is called a lookahead. Anything before the .jpg excluding the .jpg