VS2013 can't replace content with $1, why?

Question

I am using VS2013, and i want replace some text. Because the file has many alike word， so i have to use regex look ahead, look behind so search and replace.

text.txt:

abc adc

regex:

(?<=a)([a-z])(?=c)

and replace with

xxx$1xxx

But failed to replace all, Anyone konw this why?