How to Work with Regular Expressions Using R

This article describes how to use R-based regular expressions to perform functions such as filtering rows and columns in a table and replacing text. A nice reference for creating regular expressions in R can be found on R Studio's website here.

Requirements

An R variable, calculation or data set.

Method

1. grepl()

A useful function for identifying rows or columns to keep or exclude is grepl(). This function returns true if the specified text appears.

In the example below, we have a numeric table:

The basic format is grepl(text, data). You can additionally add a boolean ignore.case argument to account for inconsistent case.

Below, we will remove any row name that includes "SUM" and then keep any row where "out and about" appears:

x = No.of.colas.consumed
# remove SUMS
x = x[!grepl("SUM",rownames(x),ignore.case=T)]
# keep only out and about
tab = x[grepl("out and about",rownames(x),ignore.case=T)]

If you have more than one term to filter by, for example, rows that include "out and about" or "Coke", you can update the condition as follows:

tab = x[grepl("out and about|Coke",rownames(x),ignore.case=T)]

2. gsub()

A useful function for replacing text is gsub(). This is similar to the sub() function, but replaces all instances of the specified text.

In the example below, we have a text output:

The basic format is gsub(find_text, replace_text, data). You can additionally add a boolean ignore.case argument to account for inconsistent case.

Below we will replace all instances of "Pepsi" with "PEPSI":

x = Main.difference.between.cola.drinkers
tab = gsub("Pepsi","PEPSI",x,ignore.case=T)

If we look at the third row more closely, we can replace both "////" and "//" with a space:

tab = gsub("////|//"," ",x)

If you wish to replace special characters like ".", you will need to place "\\" in front of them:

tab = gsub("\\.",";",x)

If you wish to remove everything after "////", you would do the below:

tab = gsub("////.*","",x)

And if you wish to remove everything before "////", you would instead do the following:

tab = gsub(".*////","",x)

See Also

How to Use R in Displayr