[R] Matching a period in grep...

[Marc Schwartz]

on 08/05/2008 08:55 PM Alec.Zwart at csiro.au wrote:
> Hi folks, 
> 
> Can anyone enlighten me as to why I get the following when I search for
> ".csv" at the end of a string?
> 
>> grep("\.csv$","Blah.csv",value=TRUE)
> 
> [1] "Blah.csv"
> Warning messages:
> 1: '\.' is an unrecognized escape in a character string 
> 2: unrecognized escape removed from "[\.]csv$"
> 
> 
> R reference for regular expressions says
> 
> "Any metacharacter with special meaning may be quoted by preceding it
>  with a backslash. [...] The metacharacters in EREs are . \ | ( ) [ { ^
> $ * + ?" 
> 
> Am I missing something here?  If "\." is not the right way to match a
> period, can anyone tell me what is?  I can't find anything on this in R
> reference...

Look a little lower on the same page, where it notes:

Patterns are described here as they would be printed by cat: do remember 
that backslashes need to be doubled when entering R character strings, 
e.g. from the keyboard.

Thus:

grep("\\.csv$", "Blah.csv", value = TRUE)


Also see FAQ 7.37:

http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-does-backslash-behave-strangely-inside-strings_003f


> I'm using R 2.6 on Windows XP

Time to upgrade....2.7.1 is the current version.

HTH,

Marc Schwartz