Greedy, Non-Greedy, All-Greedy Matching in C# Regex

Question

Hi,

How can I get all the matches in the following example:

// Only "abcd" is matched
MatchCollection greedyMatches = Regex.Matches("abcd", @"ab.*");

// Only "ab" is matched
MatchCollection lazyMatches   = Regex.Matches("abcd", @"ab.*?");

// How can I get all matches: "ab", "abc", "abcd"

Thanks.

Peter

P.S.: I want to have the all matches in a generic manner. The example above is just an example. :-)

Answer 1

You can't get three different results from only one match.

If you want to match only "ab" you can use ab.? or a.{1} (or a lot of other options)
If you want to match only "abc" you can use ab. or a.{2} (or a lot of other options)
If you want to match only "abcd" you can use ab.* or a.{3} (or a lot of other options)

Answer 2

You could use something like

MatchCollection nonGreedyMatches = Regex.Matches("abcd", @"(((ab)c)d)");

Then you should have 3 backreferences with ab, abc and abcd

But to be honest, this kind of reg ex doesn't makes to much sense, especially when it gets bigger it becomes unreadable

edit:

MatchCollection nonGreedyMatches = Regex.Matches("abcd", @"ab.?");

And you got an error there btw. This can only match ab and abc (read: ab + any (optional) character

Lazy version of

MatchCollection greedyMatches    = Regex.Matches("abcd", @"ab.*");

is

MatchCollection nonGreedyMatches    = Regex.Matches("abcd", @"ab.*?");

Answer 3

If a solution exists, it probably involves a capturing group and the RightToLeft option:

string s = @"abcd";
Regex r = new Regex(@"(?<=^(ab.*)).*?", RegexOptions.RightToLeft);
foreach (Match m in r.Matches(s))
{
  Console.WriteLine(m.Groups[1].Value);
}

output:

abcd
abc
ab

I say "if" because, while it works for your simple test case, I can't guarantee this trick will help with your real-world problem. RightToLeft mode is one of .NET's more innovative features--offhand, I can't think of another flavor that has anything equivalent to it. The official documentation on it is sparse (to put it mildly), and so far there don't seem to be a lot developers using it and sharing their experiences online. So try it and see what happens.