R:在sub中引用匹配的字符串

fra*_*nkc 2 regex r

如何在R中引用匹配的字符串(或捕获的组)以便附加到sub().例如,我会这样做,以便在比赛中sed附加一个'd'

echo "66xx" | sed 's/[0-9][0-9]/&d/g'
66dxx
Run Code Online (Sandbox Code Playgroud)

Geo*_*tas 7

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