如果使用lookarounds是一种可能性为您，这里是一个解决方案。匹配以下模式：

(?<=a)a(?=a*b)

然后仅用单个b替换。模式说要匹配：

(?<=a)   assert that at least one 'a' precedes (i.e. ignore first 'a')
a        a letter 'a'
(?=a*b)  which is following by zero or more 'a' then a 'b'

这里是工作demo。

这里有一个解决方案，不限于字母'a'和'b'。
以下正则表达式的匹配项将被捕获组2的内容替换。

(?<=(.))\1(?=\1*(.))

Demo
正则表达式由以下元素组成。

(?<=    # begin a positive lookbehind
  (.)   # match any character and save to capture group 1
)       # end positive lookbehind  
\1      # match the content of capture group 1 
(?=     # begin a positive lookahead
  \1*   # match the content of capture group 1 zero or more times
  (.)   # match any character and save to capture group 2
)       # end positive lookahead