[R] string pattern matching

[William Dunlap]

You did not describe the goal of your pattern matching.  Were you trying
to match any string that could be interpreted as an R expression containing
X1 and X3 as additive terms?   If so, you could turn the string into a one-sided
formula and use the terms() function.  E.g.,

f <- function(string) {
    fmla <- as.formula(paste("~", string))
    term.labels <- attr(terms(fmla), "term.labels")
    all(c("X1","X3") %in% term.labels)
}

> f("X3 + X2 + X1")
[1] TRUE
> f("- X3 + X2 + X1")
[1] FALSE
> f("X3 + X2 + log(X1)")
[1] FALSE
> f("X3 + X2 + log(X1) + X1")
[1] TRUE
Bill Dunlap
TIBCO Software
wdunlap tibco.com


On Wed, Mar 22, 2017 at 6:39 AM, Joe Ceradini <joeceradini at gmail.com> wrote:
> Wow. Thanks to everyone (Jim, Ng Bo Lin, Bert, David, and Ulrik) for
> all the quick and helpful responses. They have given me a better
> understanding of regular expressions, and certainly answered my
> question.
>
> Joe
>
> On Wed, Mar 22, 2017 at 12:22 AM, Ulrik Stervbo <ulrik.stervbo at gmail.com> wrote:
>> Hi Joe,
>>
>> you could also rethink your pattern:
>>
>> grep("x1 \\+ x2", test, value = TRUE)
>>
>> grep("x1 \\+ x", test, value = TRUE)
>>
>> grep("x1 \\+ x[0-9]", test, value = TRUE)
>>
>> HTH
>> Ulrik
>>
>> On Wed, 22 Mar 2017 at 02:10 Jim Lemon <drjimlemon at gmail.com> wrote:
>>>
>>> Hi Joe,
>>> This may help you:
>>>
>>> test <- c("x1", "x2", "x3", "x1 + x2 + x3")
>>> multigrep<-function(x1,x2) {
>>>  xbits<-unlist(strsplit(x1," "))
>>>  nbits<-length(xbits)
>>>  xans<-rep(FALSE,nbits)
>>>  for(i in 1:nbits) if(length(grep(xbits[i],x2))) xans[i]<-TRUE
>>>  return(all(xans))
>>> }
>>> multigrep("x1 + x3","x1 + x2 + x3")
>>> [1] TRUE
>>> multigrep("x1 + x4","x1 + x2 + x3")
>>> [1] FALSE
>>>
>>> Jim
>>>
>>> On Wed, Mar 22, 2017 at 10:50 AM, Joe Ceradini <joeceradini at gmail.com>
>>> wrote:
>>> > Hi Folks,
>>> >
>>> > Is there a way to find "x1 + x2 + x3" given "x1 + x3" as the pattern?
>>> > Or is that a ridiculous question, since I'm trying to find something
>>> > based on a pattern that doesn't exist?
>>> >
>>> > test <- c("x1", "x2", "x3", "x1 + x2 + x3")
>>> > test
>>> > [1] "x1"           "x2"           "x3"           "x1 + x2 + x3"
>>> >
>>> > grep("x1 + x2", test, fixed=TRUE, value = TRUE)
>>> > [1] "x1 + x2 + x3"
>>> >
>>> >
>>> > But what if only have "x1 + x3" as the pattern and still want to
>>> > return "x1 + x2 + x3"?
>>> >
>>> > grep("x1 + x3", test, fixed=TRUE, value = TRUE)
>>> > character(0)
>>> >
>>> > I'm sure this looks like an odd question. I'm trying to build a
>>> > function and stuck on this. Rather than dropping the whole function on
>>> > the list, I thought I'd try one piece I needed help with...although I
>>> > suspect that this question itself probably does bode well for my
>>> > function :)
>>> >
>>> > Thanks!
>>> > Joe
>>> >
>>> > ______________________________________________
>>> > R-help at 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 at 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.
>
>
>
> --
> Cooperative Fish and Wildlife Research Unit
> Zoology and Physiology Dept.
> University of Wyoming
> JoeCeradini at gmail.com / 914.707.8506
> wyocoopunit.org
>
> ______________________________________________
> R-help at 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.