Regex 101 Answer I3 - Expand ranges in a string

Sorry about the lateness of this one. I had a meeting on Friday afternoon, and then had some things to take care of today. So anyway...

I3 - Expand ranges in a string

Given a string like:

1,2,4,6-9,12,15-17,20

expand the ranges, so the final string is:

1,2,4,6,7,8,9,12,15,16,17,20

Answer:

This is a fun one, because it uses a .NET regex capability that isn't in a versions of regex. If you look at the docs for Regex.Replace, you'll see that there's a version like this:

public string Replace(string, MatchEvaluator);

When the regex engine finds a match, it calls into the MatchEvaluator delegate to do the substitution. So, to match a range, we'll use a regex like:

(?<Start>\d+)\-(?<End>\d+)

 and then, we use the following method as the MatchEvaluator:

static public string Evaluator(Match match) {
int start = Int32.Parse(match.Groups["Start"].Value);
int end = Int32.Parse(match.Groups["End"].Value);
string[] values = new string[end - start + 1];
for (int i = 0; i < values.Length; i++)
{
values[i] = (start + i).ToString();
}
return String.Join(",", values);
}  

Figuring out how that works is left as an exercise to the reader.

The nice thing about using MatchEvaluator methods is that it lets you create a hybrid of regex and procedural code.