R：在sub中引用匹配的字符串

Question

如何在R中引用匹配的字符串（或捕获的组）以便附加在sub()中。例如，我会在sed中这样做，以便为匹配附加'd'

echo "66xx" | sed 's/[0-9][0-9]/&d/g'
66dxx

Answer 1

> sub("([0-9][0-9])(.*)","\\1d\\2", "66xx")
[1] "66dxx"