Substitute with an evaluated expression

In Perl, if we want to substitute with an evaluated expression with regular expression, we can do as follows.

%map = (
    "a" => "one",
    "b" => "two",
    "c" => "three",
);
$txt = "a, b, c";
$txt =~s/([a|b|c])/$map{$1}/g;

Here $map{$1} is evaluated and $txt will be one, two, three.

To be formal, the matched text s is replaced by f(s) where f() is a transformation to the text s.

In R, we use gsub() to perform substitution, however, the second argument replacement can only be a normal string that the matched text cannot be changed. In following example, \\1 can be not changed.

map = c("a" = "one", "b" = "two", "c" = "three")
txt = "a, b, c";
gsub("([a|b|c])", "_\\1_", txt)  # "a" is still "a"

## [1] "_a_, _b_, _c_"

But how can we also apply f(s) in R?

If we go back to Perl, we can use the form this is a $thing to mix code ($thing) and normal text that $thing will be evaluated later and the value will be intepolated back to the text.

This scenario is similar as in the text substitution. Actually we can modify the previous example a little bit:

$txt =~s/([a|b|c])/_$map{$1}_/g;

where in the replacement part _$map{$1}_, $map{$1} is the code and will be evaluated. The value of $map{$1} will be sent back to _$map{$1}_ to perform the normal substitution.

In R, there are also similar functions that do variable or code intepolation, such as glue::glue() or GetoptLong::qq(). Then we can do similar as in Perl to substitute with the evaluated expression.

For the text "a, b, c", to convert to "one, two, three", we can first use gsub() to replace a/b/c to a template:

library(GetoptLong)
txt2 = gsub("([a|b|c])", "@{map['\\1']}", txt)
txt2

## [1] "@{map['a']}, @{map['b']}, @{map['c']}"

then evaluate the template by e.g. GetoptLong:qq():

qq(txt2)

## [1] "one, two, three"

In following, we defined a simple function gsub_eval() according to this thought:

gsub_eval = function(pattern, replacement, x, ignore.case = FALSE, perl = FALSE,
    fixed = FALSE, useBytes = FALSE, envir = parent.frame()) {
    txt = gsub(
        pattern = pattern, 
        replacement = replacement, 
        x = x, 
        ignore.case = ignore.case, 
        perl = perl,
        fixed = fixed, 
        useBytes = useBytes
    )
    unname(sapply(txt, function(x) GetoptLong::qq(x, envir = envir)))
}

gsub_eval("([a|b|c])", "@{map['\\1']}", txt)

## [1] "one, two, three"

You can use any type of code in the template. Following is another example using gsub_eval():

gsub_eval("(\\d),(\\d)", "sum=@{\\1 + \\2}", c("number 1,2", "number 3,4"))

## [1] "number sum=3" "number sum=7"