There are many ways, Rolf. You need to look into the syntax of regular expressions. It depends on how sure you are that the formats are exactly as needed. Escaping the period with one or more backslashes is one way. Using string functions is another.
Suggestion. See if you can make a regular expression that is greedy and will match everything up to period then a period then the rest and keep the first and third parts and replace the middle with a minus sign. Or, match five things. Everything up to a single period, the period, everything between, the second period, and the rest, and keep the needed parts as above. Periods and dashes must be used carefully though. A period means match one of almost anything so a good way to catch it is [.] which matches a single character of only a period. Put parens around that: "([.])" and you have a replaceable item. In your case, you may want the parens around everything else before and after, perhaps ([^.]*[.][^.]) then [.] then ([^.]*) as one long string and replace it with \1-\2 or some similar notation. There are many other variation on this theme and some are simpler if the exact format is consistent such as 'a' being a single character or the string being a fixed length. If you are sure the period in "a.b.c" is always the fourth character, no RE is needed. Use string methods. Even if not, you can use string methods to search for a period from the end backwards or search forward once to find the first and second time starting just past it. Then replace. Fairly straightforward and very possibly much faster. -----Original Message----- From: R-help <r-help-boun...@r-project.org> On Behalf Of Rolf Turner Sent: Monday, February 8, 2021 9:29 PM To: "r-help@R-project.org" <r-help@R-project.org>"@r-project.org Subject: [R] Help with regular expressions. I want to deal with strings of the form "a.b.c" and to change (using sub() or whatever is appropriate) the second "." to a "-", i.e. to change "a.b.c" to "a.b-c". I want to leave the first "." as-is. I guess I could do a gsub(), changing all "."s to "-"s, and then do a sub() changing the first "-" back to a ".". But this seems very kludgy. There must be a sexier way. Mustn't there? Is there regular expression syntax for picking out the second occurence of a particular string? cheers, Rolf Turner -- Honorary Research Fellow Department of Statistics University of Auckland Phone: +64-9-373-7599 ext. 88276 ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see 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@r-project.org mailing list -- To UNSUBSCRIBE and more, see 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.
