How to filter a list of strings matching a pattern

Question

I have a list of strings (file names actually) and I'd like to keep only those that match a filter expression like: \*_Test.txt.

What would be the best to achieve this?

Here is the answer that I came up with:

List<string> files = new List<string>(); files.Add("MyFile1.csv"); files.Add("MyFile1_Test.txt"); files.Add("MyFile2.csv"); files.Add("MyFile2_Test.txt"); files.Add("MyFile3.csv"); files.Add("MyFile3_Test.txt"); files.Add("MyFile_Testtxttxt.txt");  // Define a filter string filter = "*_Test.txt";  // Make the filter regex safe foreach (char x in @"\+?|{[()^$.#")     filter = filter.Replace(x.ToString(), @"\" + x.ToString());  filter = string.Format("^{0}$",filter.Replace("*", ".*"));  // Old School List<string> resultList1 = files.FindAll(delegate(string s) { return Regex.IsMatch(s, filter, RegexOptions.IgnoreCase); });  // Version using LINQ List<string> resultList2 = files.Where(x =>  Regex.IsMatch(x, filter, RegexOptions.IgnoreCase) == true ).ToList(); 

Answer 1

You probably want to use a regular expression for this if your patterns are going to be complex....

you could either use a proper regular expression as your filter (e.g for your specific example it would be new Regex(@"^.*_Test\.txt$") or you could apply a conversion algorithm.

Either way you could then just use linq to apply the regex.

for example

var myRegex=new Regex(@"^.*_Test\.txt$"); List<string> resultList=files.Where(myRegex.IsMatch).ToList(); 

Some people may think the above answer is incorrect, but you can use a method group instead of a lambda. If you wish the full lamda you would use:

var myRegex=new Regex(@"^.*_Test\.txt$"); List<string> resultList=files.Where(f => myRegex.IsMatch(f)).ToList(); 

or non Linq

List<string> resultList=files.FindAll(delegate(string s) { return myRegex.IsMatch(s);}); 

if you were converting the filter a simple conversion would be

 var myFilter="*_Test.txt";  var myRegex=new Regex("^" + myFilter.Replace("*",".*") +"$"); 

You could then also have filters like "*Test*.txt" with this method.

However, if you went down this conversion route you would need to make sure you escaped out all the special regular expression chars e.g. "." becomes @".", "(" becomes @"(" etc.......

Edit -- The example replace is TOO simple because it doesn't convert the . so it would find "fish_Textxtxt" so escape atleast the .

so

string myFilter="*_Test.txt"; foreach(char x in @"\+?|{[()^$.#") {   myFilter = myFilter.Replace(x.ToString(),@"\"+x.ToString()); } Regex myRegex=new Regex(string.Format("^{0}$",myFilter.Replace("*",".*")));