How to use Regex in C# to extract multiple substrings from a string

Question

I search from the web and I have a partial solution only, so I make this question.

Input:

[A] this is A, and , [B] this is B, and hello , [C] this is C - From Here

I want to have a list:

list[0] == "this is A, and"
list[1] == "this is B, and hello"
list[2] == "this is C"
list[3] == "From Here"

I find that I should have something like this:

Regex pattern = new Regex(@"^\[A\] (.*) , \[B\] (.*) , \[C\] (.*) - (.*)$");
List<string> matches = pattern.Matches(input).OfType<Mathc>().Select(m => m.value).Distinct().ToList();

But it is not working. I would like to ask how to make it works. Thanks.

It seems the regex "works" - regex101.com/r/VuQaNJ/1. Do you only need help with the code? Or is the number of [A], [B], etc. is variable? — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Nov 24, 2020 at 14:49
You need to iterate the Groups of the resulting MatchCollection which only has one match. Note the first group will be the entire match, then each of your capture groups. — juharr
– juharr, Commented Nov 24, 2020 at 14:53

zenitex · Accepted Answer · 2020-11-24 16:24:29Z

The regex is correct, the only thing that you need to do is to iterate on the match groups. In your case the first group will be the whole sentence, so, you can simply skip the first item.
P.S. and of course don't forget to check if there is at least one match result presented. Also if this function will be executed many times I recommend you to extract regex to the static member of your class (because of performance and memory usages).

private static readonly Regex pattern = new Regex(@"^\[A\] (.*) , \[B\] (.*) , \[C\] (.*) - (.*)$");

The final version of the method (with a pattern as a static member) looks like this.

public static List<string> GetMatches(string input)
{
    var matchResult = pattern.Match(input);
    if (matchResult.Length > 0)
    {
        return matchResult.Groups.Values
            .Skip(1)
            .Select(x => x.Value)
            .ToList();
    }
    
    return new List<string>();
}

Thomas Weller · Accepted Answer · 2020-11-24 15:13:10Z

1

The problem is with a confusion between a match and a group. The regex matches only once, but it has several groups inside. Access the first match with [0], then use .OfType<Group>():

List<string> matches = pattern.Matches(input)[0].Groups.OfType<Group>().Select(m => m.Value).Distinct().ToList()

This will give you 5 results:

You can get rid of the first one with .Skip(1) or matches.RemoveAt(0);.

answered Nov 24, 2020 at 15:13

Thomas Weller

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