Regex to keep the last 4 characters of a string of unknown length

I need to use Regex to keep the last 4 characters of a string. I don't know the length of the string so I need to start at the end and count backwards. The program is written in c#.

Below are two examples:

840057

1002945

I need to be left with:

0057

2945

I have tried ^....(.*) and ^.{0,4} but this leaves me with 3 or 4 characters depending on the length of the string.

^....(.*)

^.{0,4}

When I try .{4}$ I am left with 100 and not the last 4 digits 2945. Below is a code snippet which may help.

Thank you, I appreciate the help.

4 Answers
4

You don't need to use regex for that purpose.

string MyLast4Characters = MyString.Substring(((MyString.Length >= 4) ? (MyString.Length - 4) : (0)));


That part ((MyString.Length >= 4) ? (4) : (0)) is made to check if the original string is longer or equal to 4 characters, then it will return the lasts 4 characters, else the whole string

((MyString.Length >= 4) ? (4) : (0))

.*(?=.{4})$


will match everything up to the four last characters of the string. If you replace that match with String.Empty, only those four characters remain.

String.Empty

If the string contains fewer than four characters, they will remain in the string because the regex won't match at all so there is nothing to replace.

If this has to be regex, I think you want:
.{4}(?=s|$)

.{4}(?=s|$)

But I agree that regex probably is not the best solution here.

A breakdown:

. : any character
{4} : exacty four times
(?= : followed by
s : white space
| : or
$ : a line ending
) : end the followed by section

. : any character
{4} : exacty four times
(?= : followed by
s : white space
| : or
$ : a line ending
) : end the followed by section

I guess this is something with your RegexOptions. In my example I use SingleLine mode ((?s)) and multi-line string:

RegexOptions

SingleLine

(?s)

static void RegexTest()
{
string str = "i am long stringrnwith the number 1002945";
string pattern = @"(?s)[0-9]{4}$"; // or @"(?s).{4}$"
string num = Regex.Match(str, pattern).Value;
}


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