R regular expressions

Overview

Teaching: 10 min
Exercises: 0 min

Questions

• How can we invoke regular expressions using R?

Objectives

• Introduce regex capabilities in R

Regular expressions, with search, replace capabilities, are also available in R. Again, patterns follow all the same syntax you’ve learnt using grep and sed. And again, you’ll find that the following patterns work that didn’t work with Grep:

The grep equivalent in R is a function also named grep. It’s intended for use in searching vectors of strings.
grep( 'pattern', string.vector )
Basic use returns indices of which vector entries matched the pattern.

Note that you’ll need to use double-backslashes in place of any single backslash in R.

For example, find words where the same letter is repeated consecutively:

words <- c("apple", "banana", "carrot")
grep('(\\w)\\1', words)

[1] 1 3

The indices output of R grep may be used to access the actual matched entries:

words[ grep('(\\w)\\1', words) ]

[1] "apple"  "carrot"

‘grepl’ prints a vector the same length as your string vector, with ‘true’ and ‘false’ values designating which elements matched.

grepl('(\\w)\\1', words)

[1]  TRUE FALSE  TRUE

‘regexpr’ will store span details of matching sub-strings, then ‘regmatches’ will fetch those matching sub-strings.

matchspans <- regexpr('(\\w)\\1', words)
regmatches(words, matchspans)

[1] "pp" "rr"

Substitution in R is enabled through the ‘sub’ or ‘gsub’ functions.
gsub( 'pattern', 'replacement', string.vector )
These functions will complete substitutions in all matching entries in a string vector. The difference is that gsub will also perform greedy matching and replacing within individual vector entries, where sub will only replace the first match in each vector entry. The functions return the modified string vector.

sub('(\\w)\\1', '\\1a\\1a\\1', words)

[1] "apapaple"  "banana"    "carararot"

More reference for grep and gsub with R may be found at: https://www.rdocumentation.org/packages/base/versions/3.5.1/topics/grep https://www.rdocumentation.org/packages/base/versions/3.5.1/topics/regex

Key Points

• Regular expressions through R:

• grep( 'pattern', string.vector )

• sub( 'pattern', 'replacement', string.vector )

• Use gsub instead of sub for greedy find+replace mode.

• Need to double escape \ any back slashes in patterns.