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.