>s <- "EndMemo.com R Language Tutorial"
>substr(s,0,7)

[1] "EndMemo"

Get string length:

>nchar(s)

[1] 31

To uppercase:

>x <- toupper(s)
>x

[1] "ENDMEMO.COM R LANGUAGE TUTORIAL"

To lowercase:

>x <- tolower(s)
>x

[1] "endmemo.com r language tutorial"

Split the string at letter "o":

>x <- strsplit(s,"o")

[[1]]
[1] "EndMem" ".c" "m R Language Tut" "rial"

Concatenate two strings:

>x <- paste(x," -- String Functions",sep="")
>x

[1] "endmemo.com r language tutorial -- String Functions"

Substring replacement:

>x <- sub("Tutorial","Examples",s)
>x

[1] "EndMemo.com R Language Examples"

Use regular expression:

>x <- sub("n.+e","XXX",s)
>x

[1] "EXXX Tutorial"

Please see grep() function for more regular expression handling of string.