Re: [R] regular expression help A backreference is contained in parentheses and there are no parentheses in your regular expression, hence the error message. Its probably easiest just to remove all non-letters: x <- "45x53yy66" gsub("[^[:alpha:]]", "", x) # xyy On 4/18/07, Petr PIKAL  wrote: > Dear all > > as usual I am again lost in virtues of regular expressions. > > I have such character vector named vzor: > > [365] "61A"      "62C/27"   "65A/27"   "66C/29"   "69A/29"   "70C/31" > "73A/31"   "74C/33"   "77A/33"   "81A/35"   "82C/37"   "85A/37"   "86C/39" > > [378] "89A/39"   "90C/41"   "93A/41"   "94C/43"   "97A/43"   "98C/45" > "101A/45"  "102C/47"  "105A/47"  "106C/49"  "109A/49"  "110C/51" "113A/51" > > > and I want only letters from it. > > I tried > > > gsub("[[:alpha:]]"," \\\\1",vzor) > Error in gsub("[[:alpha:]]", " \\\\1", vzor) : >        invalid backreference 1 in regular expression > > gsub("[:alpha:]"," \\\\1",vzor) > > gives me the same vector > > There is probably very simple solution to it which I overlooked and > examples in help page did not help me to find it. > > Thank you > Best regards > > Petr Pikal > petr.pikal@precheza.cz > > ______________________________________________ > R-help@stat.math.ethz.ch mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. > ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.