修改正则表达式组优先级(Modifying regex group priority)

 我有以下正则表达式： 
 
 Regex regex = new Regex(@"(?<g1>a?)(?<g2>a?)(?<g3>b?)(?<g4>b?)"); 
 
 和一个字符串 
 
string str = @"ab";
 
 
 将此正则表达式应用于我得到的字符串时 
 
 g1 -> "a", g2 -> "", g3 -> "b",  g4 -> ""
 
 
 是否可以修改此正则表达式来获取 
 
 g1 -> "a", g2 -> "", g3 -> "",  g4 -> "b"
 ？ 那就是我希望g4的优先级高于g3。 

I have the following Regex:
 
Regex regex = new Regex(@"(?<g1>a?)(?<g2>a?)(?<g3>b?)(?<g4>b?)");
 
and a string 
 
string str = @"ab";
 
 
When applying this regex to the string I get 
 
 g1 -> "a", g2 -> "", g3 -> "b",  g4 -> ""
 
 
Is it possible to modify this regex to get 
 
 g1 -> "a", g2 -> "", g3 -> "",  g4 -> "b"
? That is I want to have higher priority for g4 than for g3. 

原文：https://stackoverflow.com/questions/2085553