gsubfn {gsubfn} | R Documentation |
Like gsub
except instead of a replacement string one
uses a function which accepts the matched text as input and emits
replacement text for it.
gsubfn(pattern, replacement, x, backref, USE.NAMES = FALSE, env = parent.frame(), ...)
pattern |
Same as pattern in gsub |
replacement |
A character string, function, list, formula or proto object. See Details. |
x |
Same as x in gsub |
backref |
Number of backreferences to be passed to function. If zero or positive the match is passed as the first argument to the replacement function followed by the indicated number of backreferences as subsequent arguments. If negative then only the that number of backreferences are passed but the match itself is not. If omitted it will be determined automatically, i.e. it will be 0 if there are no backreferences and otherwise it will equal negative the number of back refenrences. It determines this by counting the number of non-escaped left parentheses in the pattern. |
USE.NAMES |
See USE.NAMES in sapply . |
env |
Environment in which to evaluate the replacement function. Normally this is left at its default value. |
... |
Other gsub arguments. |
If replacement
is a string then it acts like gsub
.
If replacement
is a function then each matched string
is passed to the replacement function and the output of that
function replaces the matched string in the result. The first
argument to the replacement function is the matched string
and subsequent arguments are the backreferences, if any.
If replacement
is a list then the result of the
regular expression match is, in turn,
matched against the names of that list and the value
corresponding to the first name in the list that is match is returned.
If there are
no names matching then the first unnamed component is returned
and if there are no matches then the string to be matched is returned.
If backref
is not specified or is specified and is
positive then the entire match is used to lookup the value in the list
whereas if backref
is negative then the identified backreference is
used.
If replacement
is a formula instead of a function then
a one line function is created whose body is the right hand side
of the formula and whose arguments are the left hand side separated
by +
signs (or any other valid operator). The environment
of the function is the environment of the formula. If the arguments
are omitted then the free variables found on the right hand side
are used in the order encountered. 0
can be used to indicate
no arguments. letters
, LETTERS
and pi
are
never automatically used as arguments.
If replacement
is a proto object then it should have a
fun
method which is like the replacement function except
its first argument is the object and the remaining arguments are
as in the replacement function and are affected by backref in
the same way. gsubfn
automatically inserts the named arguments
in the call to gsubfn
into the proto object and also
maintains a count
variable which counts matches within
strings. The user may optionally specify pre
and post
methods in the proto object which are fired at the beginning and
end of each string (not each match). They each take one argument,
the object.
Two utility functions cat0
and paste0
are available.
They are like
cat
and paste
except that their default
code{sep} value is ""
.
As in gsub
.
# adds 1 to each number in third arg gsubfn("[[:digit:]]+", function(x) as.numeric(x)+1, "(10 20)(100 30)") # same but using formula notation for function gsubfn("[[:digit:]]+", ~ as.numeric(x)+1, "(10 20)(100 30)") # replaces pairs m:n with their sum s <- "abc 10:20 def 30:40 50" gsubfn("([0-9]+):([0-9]+)", ~ as.numeric(x) + as.numeric(y), s) # default pattern for gsubfn does quasi-perl-style string interpolation gsubfn( , , "pi = $pi, 2pi = `2*pi`") # Extracts numbers from string and places them into numeric vector v. # Normally this would be done in strapply instead. v <- c(); f <- function(x) v <<- append(v,as.numeric(x)) junk <- gsubfn("[0-9]+", f, "12;34:56,89,,12") v # same strapply("12;34:56,89,,12", "[0-9]+", simplify = c) # makes all letters except first in word lower case gsubfn("\\B.", tolower, "I LIKE A BANANA SPLIT", perl = TRUE) # replaces numbers with that many Xs gsubfn("[[:digit:]]+", ~ paste(rep("X", n), collapse = ""), "5.2") # replaces units with scale factor gsubfn(".m", list(cm = "e1", km = "e6"), "33cm 45km") # place <...> around first two occurrences p <- proto(fun = function(this, x) if (count <= 2) paste0("<", x, ">") else x) gsubfn("\\w+", p, "the cat in the hat is back") # replace each number by cumulative sum to that point p2 <- proto(pre = function(this) this$value <- 0, fun = function(this, x) this$value <- value + as.numeric(x)) gsubfn("[0-9]+", p2, "12 3 11, 25 9")