Regex replace everything except a particular pattern

Question

I\'m looking to extract:

50%

From a string that will have more or less this format:

The 50% is in here somewhere.

I\'d

Answer 1

One solution is to use regex replace as follows:

Regex.Replace("50% of 50% is 25%", "(\d+\%)|(?:.+?)", "$1");

Output:

50%50%25%

As a general approach:

Regex.Replace(input, (pattern)|(?:.+?), "$1");

This finds anything that matches either of the following:

• The pattern. Captured as $1. This is what we want to keep.
• Any character, any number of times, but non-greedy. This finds anything that is not captured by the first group. ?: because we don't need to capture this group.

As MSDN states: "$1 replaces the entire match with the first captured subexpression." (That is, all matches for that substring, concatenated.)

Effectively, this is the described regex filter.

Answer 2

You can use Regex.Matches and concatenate each matches result. Just pick one you like the most.

//Sadly, we can't extend the Regex class
public class RegExp
{
    //usage : RegExp.Filter("50% of 50% is 25%", @"[0-9]+\%")
    public static string Filter(string input, string pattern)
    {
        return Regex.Matches(input, pattern).Cast<Match>()
            .Aggregate(string.Empty, (a,m) => a += m.Value);
    }
}

public static class StringExtension
{
    //usage : "50% of 50% is 25%".Filter(@"[0-9]+\%")
    public static string Filter(this string input, string pattern)
    {
        return Regex.Matches(input, pattern).Cast<Match>()
            .Aggregate(string.Empty, (a,m) => a += m.Value);
    }
}

Answer 3

I do not understand your reasoning why you want to use replace. Why go that way in the first place? There are methods in the Regex class that allow you to precisely get all the desired matches. Your roundabout way at getting to your solution I find is pointless.

Just use Matches() to collect the matches. You could then join them into the string that you wanted.

var str = "50% of 50% is 25%";
var re = new Regex(@"\d+%");
var ms = re.Matches(str);
var values = ms.Cast<Match>().Select(m => m.Value);
var joined = String.Join("", values); // "50%50%25%"