Modifying existing strings via substitution is a common practice in programing. To this end, functions like gsub provide a method to accomplish this. Below is an example where “hey” is replaced with “ho” transforming a line from the Ramones into Santa Claus leaving on Christmas Eve.
s = "hey ho, let's go!"
gsub("hey","ho",s)## [1] "ho ho, let's go!"gsub only supports one string of matching with one string of replacement. What this means is while you can match on multiple conditions, you can only provide one condition of replacement. Below we construct a regular expression which matches on “hey” or “ho” and replaces any such matches with “yo”.
s = "hey ho, let's go!"
gsub("hey|ho","yo",s)## [1] "yo yo, let's go!"If you wanted to replace “hey” with “get” and “ho” with “ready” you would need two steps.
s = "hey ho, let's go!"
s_new = gsub("hey","get",s)
s_new = gsub("ho","ready",s_new)
s_new## [1] "get ready, let's go!"This sequential process however can result in undesired changes. If we want to swap where “hey” and “ho” are, we can see the process breaks down. Because each change happens in order, “hey” becomes “ho” and then every “ho” becomes “hey”, undoing the first step.
s = "hey ho, let's go!"
s_new = gsub("hey","ho",s)
s_new = gsub("ho","hey",s_new)
s_new## [1] "hey hey, let's go!"This is where the idea of mgsub comes in. mgsub is a safe, simultaneous string substitution function. We pass in a named list (or named vector) of changes to make along with the string and the replacements are simultaneous. The names are the matching criteria and the values are the replacements.
library(mgsub)
s = "hey ho, let's go!"
mgsub::mgsub(s,list("hey"="ho","ho"="hey"))## [1] "ho hey, let's go!"Below you can see an example of using named vector. where the matches and replacements start out as independent vectors.
s = "hey ho, let's go!"
replacements = c("ho","hey")
matches = c("hey","ho")
names(replacements) = matches
mgsub::mgsub(s,replacements)## [1] "ho hey, let's go!"Below you can see an example of using a named vector where the names are defined inline.
s = "hey ho, let's go!"
mgsub::mgsub(s,c("hey"="ho","ho"="hey"))## [1] "ho hey, let's go!"mgsub fully supports regular expressions as matching criteria as well as backreferences in the replacement. Note how the matching criteria ignores “dopachloride” for replacement but matches both “Dopazamine” and “dopastriamine” (all fake chemicals despite what the replace string claims!).
s = "Dopazamine is not the same as dopachloride or dopastriamine, yet is still fake."
mgsub::mgsub(s,list("[Dd]opa([^ ]*?mine)"="Meta\\1","fake"="real"))## [1] "Metazamine is not the same as dopachloride or Metastriamine, yet is still real."Furthermore, you can pass through any options from the gsub family. In the example below you can see fixed string matching
s = "All my life I chased $money$ and .power. - not love!"
mgsub::mgsub(s,list("$money$"="balloons",".power."="dolphins","love"="success"),fixed=TRUE)## [1] "All my life I chased balloons and dolphins - not success!"This is actually the most compelling feature of mgsub. The package qdap implements a similar type function (also named mgsub) which does not employ safe substitution. Here safe means a few things:
First, a demonstration of the first form of safety. Note how we are searching for ‘they’ and ‘the’ where ‘the’ is a substring of ‘they’. If ‘the’ is matched before ‘they’, we would expect to see “ay don’t understand the value of what they seek.”, but in both cases, the replacements occur correctly.
s="they don't understand the value of what they seek."
mgsub::mgsub(s,list("the"="a","they"="we"))## [1] "we don't understand a value of what we seek."qdap::mgsub(c("the","they"),c("a","we"),s)## [1] "we don't understand a value of what we seek."We can continue to test this by using variable length regular expression matches. Note that we provide two different matching criteria, one a regular expression of length 6 but which matches a length 10 and the other a match of length 9. However, qdap only prioritizes based on the length of the regular expression, not on the actual length of the match. While this is an edge case, it an example of safety provided by mgsub.
s="Dopazamine is a fake chemical"
mgsub::mgsub(s,list("dopazamin"="freakout","do.*ne"="metazamine"),ignore.case=TRUE)## [1] "metazamine is a fake chemical"qdap::mgsub(c("dopazamin","do.*ne"),c("freakout","metazamine"),s,fixed = FALSE,ignore.case=TRUE)## [1] "freakoute is a fake chemical"In the second case, mgsub does not utilize placeholders and therefore guarantees no string collisions when replacing. Consider a simple example of shifting each word in the following string one spot to the left. mgsub correctly shifts each word while qdap provides two wrong sets of substitutions depending on the other arguments you provide.
s="hey, how are you?"
mgsub::mgsub(s,list("hey"="how","how"="are","are"="you","you"="hey"))## [1] "how, are you hey?"print(qdap::mgsub(c("hey","how","are","you"),c("how","are","you","hey"),s))## [1] "how, are you how?"print(qdap::mgsub(c("hey","how","are","you"),c("how","are","you","hey"),s
,fixed=FALSE,ignore.case=TRUE))## [1] "hey, hey hey hey?"mgsub pays the price of safety in performance, potentially running slower than qdap.
library(microbenchmark)
s = "Dopazamine is not the same as Dopachloride and is still fake."
m = c("[Dd]opa(.*?mine)","fake")
r = c("Meta\\1","real")
names(r) = m
microbenchmark(
mgsub = mgsub::mgsub(s,r),
qdap = qdap::mgsub(m,r,s,fixed=FALSE)
)## Unit: microseconds
## expr min lq mean median uq max neval
## mgsub 193.277 241.049 375.2300 294.8385 389.6525 2655.907 100
## qdap 173.584 200.753 249.6405 233.3905 273.3220 741.014 100