[R] How to handle "~" character after csv importation

[Sébastien]

Dear R users,

I have to import some csv files in which column headers contain the 
character "~". Following the following import call, the character seems 
to be replaced by dots in the column names of my data frame. Plus, I 
cannot query names(mydata) to find the column index which header should 
contain "~" or "."

 > mydata <- data.frame(read.table("mycsv.csv", sep = ",", header = 
TRUE, na.strings = ".", as.is = TRUE))
 > mydata
  P1 P2 P3 P1.P2 P1.P3 P2.P3 A B C
1  1  2  3     4     5     6 7 8 9
 > names(mydata)
[1] "P1"    "P2"    "P3"    "P1.P2" "P1.P3" "P2.P3" "A"     "B"     "C"   
 > grep("~",names(mydata))
integer(0)
 > grep(".",names(mydata))
[1] 1 2 3 4 5 6 7 8 9

How can I check for this character ? And by the way, can anyone explain 
to me the result of the last grep call?

Thank you in advance.

Sebastien