Question

The following code return 1:

Regex.Match("aaa", "(a)").Groups[1].Captures.Count

But I expect to receive 3: I see three captures of a.

Answer 1

You need to either get the match count:

Regex.Matches("aaa", "(a)").Count

Or add a quantifier to the regex:

Regex.Match("aaa", "(a)+").Groups[1].Captures.Count

The regex (a) matches only a single a. In the first example above, that regex can be matched three times.

In the second example the regex matches several as at once and captures each one into group 1.

To make a choice you should consider the following differences between them:

Regex.Matches("aaba", "(a)").Count // this is 3
Regex.Match("aaba", "(a)+").Groups[1].Captures.Count // this is 2

The second only yields two captures because it matches the first sequence of two as but then it stops matching when it finds a b. The + quantifier only matches unbroken sequences.