Modifying strings by substitution

Introduction#

sub and gsub are used to edit strings using patterns. See Pattern Matching and Replacement for more on related functions and Regular Expressions for how to build a pattern.

Rearrange character strings using capture groups

If you want to change the order of a character strings you can use parentheses in the pattern to group parts of the string together. These groups can in the replacement argument be addresed using consecutive numbers.

The following example shows how you can reorder a vector of names of the form “surname, forename” into a vector of the form “forename surname”.

library(randomNames) 
set.seed(1)

strings <- randomNames(5)
strings
# [1] "Sigg, Zachary"        "Holt, Jake"           "Ortega, Sandra"       "De La Torre, Nichole"
# [5] "Perkins, Donovon"  

sub("^(.+),\\s(.+)$", "\\2 \\1", strings)
# [1] "Zachary Sigg"        "Jake Holt"           "Sandra Ortega"       "Nichole De La Torre"
# [5] "Donovon Perkins"

If you only need the surname you could just address the first pairs of parentheses.

sub("^(.+),\\s(.+)", "\\1", strings)
# [1] "Sigg"        "Holt"        "Ortega"      "De La Torre" "Perkins"

Eliminate duplicated consecutive elements

Let’s say we want to eliminate duplicated subsequence element from a string (it can be more than one). For example:

2,14,14,14,19

and convert it into:

2,14,19

Using gsub, we can achieve it:

gsub("(\\d+)(,\\1)+","\\1", "2,14,14,14,19")
[1] "2,14,19"

It works also for more than one different repetition, for example:

 > gsub("(\\d+)(,\\1)+", "\\1", "2,14,14,14,19,19,20,21")
[1] "2,14,19,20,21"

Let’s explain the regular expression:

(\\d+): A group 1 delimited by () and finds any digit (at least one). Remember we need to use the double backslash (\\) here because for a character variable a backslash represents special escape character for literal string delimiters (\" or \'). \d\ is equivalent to: [0-9].
,: A punctuation sign: , (we can include spaces or any other delimiter)
\\1: An identical string to the group 1, i.e.: the repeated number. If that doesn’t happen, then the pattern doesn’t match.

Let’s try a similar situation: eliminate consecutive repeated words:

one,two,two,three,four,four,five,six

Then, just replace \d by \w, where \w matches any word character, including: any letter, digit or underscore. It is equivalent to [a-zA-Z0-9_]:

> gsub("(\\w+)(,\\1)+", "\\1", "one,two,two,three,four,four,five,six")
[1] "one,two,three,four,five,six"
>

Then, the above pattern includes as a particular case duplicated digits case.