Check if string ends with several digits after some specific char in C#

Question

I have some list that is holding several strings, for example:

List<string> list1 = new List<string>()
        {
            "REGISTER_OPTION_P2", "REGISTER_OPTION_P27", "REGISTER_OPTION_P254","REGISTER_OPTION_NOFW", "POWER_OPTION_P45JW"
        };

I Want to filter all the strings that are ending with the _P*where * is several digits only and not non-digits.

The result for the above will hold the following:

"REGISTER_OPTION_P2", "REGISTER_OPTION_P27", "REGISTER_OPTION_P254"

I know there is char.IsDigit() but it operates only on 1 digit. My case is multiple digits.

Any option to make it?

Have you explored regular expressions? Is there a maximum and minimum number of digits after the P? Will the P always be after the last underscore? — gunr2171, Sep 19 '22 at 13:30

Wiktor Stribiżew · Accepted Answer · 2022-09-19T13:35:05.840

4

You can use

var lst = new[] {"REGISTER_OPTION_P2", "REGISTER_OPTION_P27", "REGISTER_OPTION_P254","REGISTER_OPTION_NOFW", "POWER_OPTION_P45JW"};
var pattern = @"_P[0-9]*$";
var result = lst.Where(x => Regex.IsMatch(x, pattern, RegexOptions.RightToLeft));
foreach (var s in result)
    Console.WriteLine(s);

Output:

REGISTER_OPTION_P2
REGISTER_OPTION_P27
REGISTER_OPTION_P254

See the C# demo.

Details:

_P - a fixed string
[0-9]* - zero or more digits
$ - end of string.

Note the use of RegexOptions.RightToLeft that greatly enhances matching at the end of the string.

edited Sep 19 '22 at 13:35

answered Sep 19 '22 at 13:33

Wiktor Stribiżew

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though 0-9 aren't the only digits in this world maybe consider `\d` instead (OP didn't spedify the limitation to only 0-9) – Rand Random Sep 19 '22 at 13:44
2

@RandRandom Following the [`\d` less efficient than \[0-9\]](https://stackoverflow.com/q/16621738/3832970), I tend to use explicit digit patterns when it comes to ASCII digits (as shown in the examples). If any Unicode digits like `۱`, `௫` or `৫` can appear after `_P`, yes, `\d*` would be best. – Wiktor Stribiżew Sep 19 '22 at 13:48

Eric Yang · Answer 2 · 2022-09-19T13:35:53.820

2

So the regex expression that will catch that is

P(\d+$)

\d stands for digit, + is more than 1, $ is the end of the string, and () specifies that it should be captured. C# should have a findAll function in regex.

One tool that is really helpful for me (because I'm not great at regex) is

https://www.autoregex.xyz/

edited Sep 19 '22 at 13:35

answered Sep 19 '22 at 13:33

Eric Yang

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Might want to include the P in the pattern. This also does not work with 0 digits. – gunr2171 Sep 19 '22 at 13:33

John Alexiou · Answer 3 · 2022-09-19T13:57:02.980

2

Use the String.Replace() function

"REGISTER_OPTION_P42".Replace("REGISTER_OPTION_P",string.Empty) = "42"

or use the String.Substring() function

"REGISTER_OPTION_P42".Substring(17) = "42"

and then use .All( (c)=>char.IsDigit(c) ) to check that all remaining characters are digits.

sample code

static void Main(string[] args)
{
    var list = new List<string>(new string[] { "REGISTER_OPTION_P23", "REGISTER_OPTION_P823", "REGISTER_OPTION_P1Q6", "REGISTER_OPTION_P5" });

    var filtered = list.Where((s) => s.Replace("REGISTER_OPTION_P", string.Empty).All((c)=>char.IsDigit(c))).ToList();

    foreach (var item in filtered)
    {
        Console.WriteLine(item);
    }
    //REGISTER_OPTION_P23
    //REGISTER_OPTION_P823
    //REGISTER_OPTION_P5
}

edited Sep 19 '22 at 13:57

answered Sep 19 '22 at 13:35

John Alexiou

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1

Clever solution, though there's the obvious downside that this will only work with a fixed prefix. If that works for the OP, great. – gunr2171 Sep 19 '22 at 13:39
1

`int.TryParse` has some downsides: https://dotnetfiddle.net/xRMVCf – Rand Random Sep 19 '22 at 13:53
1

@RandRandom - I see that, and I updated the answer to use `.All((c)=>char.IsDigit(c))` for the checking. – John Alexiou Sep 19 '22 at 14:00

Check if string ends with several digits after some specific char in C#

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