Pattern matching and replacement in R

问题

I am not familiar at all with regular expressions, and would like to do pattern matching and replacement in R.

I would like to replace the pattern #1, #2 in the vector: original = c("#1", "#2", "#10", "#11") with each value of the vector vec = c(1,2).

The result I am looking for is the following vector: c("1", "2", "#10", "#11") I am not sure how to do that. I tried doing:

for(i in 1:2) {
    pattern = paste("#", i, sep = "")
    original = gsub(pattern, vec[i], original, fixed = TRUE)
}

but I get :

#> original
#[1] "1"  "2"  "10" "11"

instead of: "1" "2" "#10" "#11"

I would appreciate any help I can get! Thank you!

回答1:

Another option using gsubfn:

library(gsubfn)
gsubfn("^#([1-2])$",  I, original)   ## Function substituting
[1] "1"   "2"   "#10" "#11"

Or if you want to explicitly use the values of your vector , using vec values:

gsubfn("^#[1-2]$",  as.list(setNames(vec,c("#1", "#2"))), original)

Or formula notation equivalent to function notation:

gsubfn("^#([1-2])$",  ~ x, original)   ## formula substituting

回答2:

Specify that you are matching the entire string from start (^) to end ($).

Here, I've matched exactly the conditions you are looking at in this example, but I'm guessing you'll need to extend it:

> gsub("^#([1-2])$", "\\1", original)
[1] "1"   "2"   "#10" "#11"

So, that's basically, "from the start, look for a hash symbol followed by either the exact number one or two. The one or two should be just one digit (that's why we don't use * or + or something) and also ends the string. Oh, and capture that one or two because we want to 'backreference' it."

回答3:

Here's a slightly different take that uses zero width negative lookahead assertion (what a mouthful!). This is the (?!...) which matches # at the start of a string as long as it is not followed by whatever is in .... In this case two (or equivalently, more as long as they are contiguous) digits. It replaces them with nothing.

gsub( "^#(?![0-9]{2})" , "" , original , perl = TRUE )
[1] "1"   "2"   "#10" "#11"

来源：https://stackoverflow.com/questions/20219311/pattern-matching-and-replacement-in-r

标签

regex

gsub