需要幫助使用正則表達式函數/修剪我的字符串

如果您不需要它是 REGEX，這應該可以工作

var rawString = "15211";
var maxLength = 4;
var trimmedResult = new string(
        rawString.TakeWhile(char.IsNumber) // loop through characters using LINQ. since your string starts with numbers, you can just keep taking until you bump into a character
            .Take(maxLength) // limit the length for cases like "15211" where you don't need the extra numbers in the end
            .ToArray() // so we can use the new string() constructor
        );