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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 a s 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.
        
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