R: Pattern Matching and Replacement

gsubfn {gsubfn} | R Documentation

## Pattern Matching and Replacement

### Description

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.

### Usage

    gsubfn(pattern, replacement, x, backref, USE.NAMES = FALSE,
    	ignore.case = FALSE, engine = getOption("gsubfn.engine"),
    	env = parent.frame(), ...)

### Arguments

`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 references. It determines this by counting the number of non-escaped left parentheses in the pattern. Also if the function contains an ampersand as an argument then `backref` will be taken as non-negative and the ampersand argument will get the full match.
`USE.NAMES` | See `USE.NAMES` in `sapply`.
`ignore.case` | If `TRUE` then case is ignored in the `pattern` argument.
`engine` | Specifies which engine to use. If the R installation has `tcltk` capability then the `tcl` engine is used unless `FUN` is a proto object or `perl=TRUE` in which case the `"R"` engine is used (regardless of the setting of this argument).
`env` | Environment in which to evaluate the replacement function. Normally this is left at its default value.
`...` | Other `gsub` arguments.

### Details

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.

Note that if the `"R"` engine is used and if backref is non-negative then internally the pattern will be parenthesized.

A utility function `cat0` is available. They are like `cat` and `paste` except that their default `sep` value is `""`.

### Value

As in `gsub`.

### See Also

`strapply`

### Examples

    # 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)

    # replaces numbers with that many Xs separated by -
    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")

    # this only works if your R installation has tcltk capabilities
    # See following example for corresponding code with R engine
    if (isTRUE(capabilities()[["tcltk"]])) {
    	gsubfn("(.)\\1", ~ paste0(`&`, "!"), "abbcddd")
    }

    # with R and backref >=0 (implied) the pattern is internally parenthesized
    # so must use \2 rather than \1
    gsubfn("(.)\\2", ~ paste0(`&`, "!"), "abbcddd", engine = "R")

[Package gsubfn version 0.6-6 Index]