C# 正则表达式匹配最后一个下划线后的最后一位数字

Question

Using regex I'm trying to get only the last digit (can be only 2 or 3) after the last underscore.

What I have right now is getting the digit and characters. I need to cut off the characters and only get the digit [2-3].

Here is my example -- I need to get only 2 after the last underscore. Currently getting both digit and characters

ABC_0000_DEFG_1I_23_45_HIJKL2.pdf

The output I want -- 2 (after HIJKL).

^.*_\K[^.]+

If I get rid of ^ with \d, d{2-3}, ... it still gets HIJKL.

Answer 1

You can use

_[^_]*(\d)[^_]*$

Which matches the last underscore, followed by a digit surrounded by anything but underscores.

You can use [23] instead of \\d if you want to ignore anything other than 2 or 3 .

Answer 2

The regular expression

_[^_]*([2-3])[^_]*$

should do you. It matches:

• _ — an underscore, followed by
• [^_]* — zero or more characters other than underscore, followed by
• ([23]) — the decimal digits 2 or 3 , followed by
• [^_]* — zero or more characters other than underscore, followed by
• $ — end-of-text

You will need to get match group #1:

var rx = new Regex(@"_[^_]*([2-3])[^_]*$");
var m  = rx.Match("ABC_0000_DEFG_1I_23_45_HIJKL2.pdf");
var s  = m.Success ? m.Groups(1) : null;

At which point, s should be "2".

Answer 3

To get a match only in .NET you might also use lookarounds:

(?<=_[^_]*)[23](?=[^_]*$)

The pattern matches:

• (?<=_[^_]*) Positive lookbehind, assert _ followed by optional chars other than _
• [23] Match either 2 or 3
• (?=[^_]*$) Positive lookahead to assert no _ till the end of the string

See a .NET regex demo or a C# demo .

Example code

Regex regex = new Regex(@"(?<=_[^_]*)[23](?=[^_]*$)");
Match match = regex.Match("ABC_0000_DEFG_1I_23_45_HIJKL2.pdf");
if (match.Success)
{
    Console.WriteLine(match.Value);
}

Output

2