C＃将两个字符串列表与一个模式相交

Question

I have two list of strings like this:

var entities= new List<string>(){"101", "102", "103",}; 
var files= new List<string>(){"101_F05_20101001.csv", "102_F05_20101001.csv", "201_F05_20101001.csv", "202_F05_20101001.csv"};

I want to get the result of intersecting them following this pattern:

ID_F05_YYYYMMDD.csv

Where ID should match one or more items in the entities list.

I have written the following code:

var list = files
    .Where(x => entities.Any(y => x.Contains(y) && x.Substring(0, y.Length) == y))
    .ToList();

Running code here .

But I wonder if this can be improved using a regex like this one:

var regex = new Regex(@"^(\d*)_F05_\d*\.csv$");

Is it possible?

Answer 1

You may use

.Where(x => Regex.IsMatch(x, $@"^(?:{string.Join("|", entities)})_F05_\d*\.csv$"))

The regex will look like ^(?:101|102|103)_F05_\\d*\\.csv$ given your current input data and it will match

• ^ - start of string
• (?:101|102|103) - a non-capturing group that matches 101 , 102 or 103
• _F05_ - a literal string
• \\d* - 0 or more digits
• \\.csv - .csv string
• $ - end of string

Note you do not need to regex escape the entities if they are numeric. Else, you need to use string.Join("|", entities.Select(Regex.Escape)) .

C# code demo :

var entities= new List<string>(){"101", "102", "103",}; 
var files= new List<string>(){"101_F05_20101001.csv", "102_F05_20101001.csv", "201_F05_20101001.csv", "202_F05_20101001.csv"};

var pat = $@"^(?:{string.Join("|", entities)})_F05_\d*\.csv$";

var list = files
        .Where(x => Regex.IsMatch(x, pat))
        .ToList();

foreach (var s in list) {
    Console.WriteLine(s);
}

Output:

101_F05_20101001.csv
102_F05_20101001.csv

Answer 2

I choose to use a little bit more linq to solve it:

        var entities = new List<string>() { "101", "102", "103", };
        var files = new List<string>() { "101_F05_20101001.cvs", "102_F05_20101001.cvs", "201_F05_20101001.cvs", "202_F05_20101001.cvs" };
        var regex = new Regex(@"^(\d*)_F05_\d*\.cvs$");

        var result = entities.SelectMany(e => files.Select(f =>
        {
            var match = regex.Match(f);
            if (match.Success)
            {
                if (match.Groups.Count > 1)
                {
                    if (match.Groups[1].Value == e) return f;
                }
            }

            return "";
        })).Where(s => !String.IsNullOrEmpty(s));