sub returns incorrect data

Question

I have the following chunk of code:

temp <- "44C"
sub("^([-+]?[0-9]+)([CF])$","\\2",temp)

This correctly returns C .

Yet when I try out

temp <- "44"
sub("^([-+]?[0-9]+)([CF])$","\\2",temp)

I was expecting an empty vector. Instead I get " 44 ".

Am I reasoning something wrong?

Answer 1

There is no \\2 in your second case.So it cannot replace anything and returns the original string unaltered. When a regex fails in sub then the original string is returned.

Answer 2

It will work if you add ? to your regex:

temp <- c("44C", "44")
sub("^([-+]?[0-9]+)([CF])?$","\\2",temp)
# [1] "C" ""