Safe Substitution

Regular Expression Support

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."
pattern = c("[Dd]opa([^ ]*?mine)", "fake")
replacement = c("Meta\\1", "real")
mgsub::mgsub(s, pattern, replacement)

## [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!"
pattern = c("$money$", ".power.", "love")
replacement = c("balloons", "dolphins", "success")
mgsub::mgsub(s, pattern, replacement, fixed = TRUE)

## [1] "All my life I chased balloons and dolphins - not success!"

Safe Substitution

This is actually the most compelling feature of mgsub. Several packages implement a similar type function (also named mgsub) which do not employ safe substitution - qdap, bazar and textclean. A detailed analysis of safety can be found on my blog. Here is a quick overview of what is meant by safety:

1. Longer matches are preferred over shorter matches for substitution first
2. No placeholders are used so accidental string collisions don’t occur

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."
pattern = c("the", "they")
replacement = c("a", "we")
mgsub::mgsub(s, pattern, replacement)

## [1] "we don't understand a value of what we seek."

qdap::mgsub(pattern, replacement, 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"
pattern = c("dopazamin", "do.*ne")
replacement = c("freakout", "metazamine")
mgsub::mgsub(s, pattern, replacement, ignore.case = TRUE)

## [1] "metazamine is a fake chemical"

qdap::mgsub(pattern, replacement, 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?"
pattern = c("hey", "how", "are", "you")
replacement = c("how", "are", "you", "hey")
mgsub::mgsub(s, pattern, replacement)

## [1] "how, are you hey?"

print(qdap::mgsub(pattern, replacement, s))

## [1] "how, are you how?"

print(qdap::mgsub(pattern, replacement, s, fixed = FALSE, ignore.case = TRUE))

## [1] "hey, hey hey hey?"