[R] regular expression for nth character in a string

[Gonçalo Ferraz]

Hi, I have a string

"InTrouble"

and want to extract, say, the first two characters: "In"
or the last three: "blee"
or the 3rd, 4th, and 5th: "Trou"

Is there an easy way of doing this quickly with regular expressions in gsub, grep or similar?

Thank you for any help.

Gonçalo