Match number between characters

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

开发者_StackOverflow中文版

How to match the number between _ and .jpg?

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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

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Use this to segregate your numbers into the group $1 gif etc. are in $2

_(\d+)\.(gif|jpg)

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This fairly simple regular expression should match the number you need as the first group:

[0-9]*_([0-9])*\.(?:jpg|gif)

It works for both .jpg and .gif files, since you used both in your examples.