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R String Search

Pattern searching tests each string in a vector against a pattern. grep and grepl answer that test in different forms.

Positions of matches

grep looks for ap inside each word. The output gives the positions of the two matching words.

words <- c("apple", "apricot", "banana", "plum")
print(grep("ap", words))

Words instead of positions

The value argument asks for the matching words themselves. The output holds the words rather than their positions.

words <- c("apple", "apricot", "banana", "plum")
print(grep("ap", words, value = TRUE))

A logical answer for each word

grepl tests each word for an. The output holds one TRUE or FALSE value for each word.

words <- c("apple", "apricot", "banana", "plum")
print(grepl("an", words))

Anchors at the start and end

The ^ mark ties ap to the start of a word. The $ mark ties e to the end of a word.

words <- c("apple", "apricot", "banana", "plum")
print(grep("^ap", words))
print(grep("e$", words, value = TRUE))

A set of characters and no match

The bracket pattern matches any one vowel. The second search looks for z and finds nothing.

words <- c("apple", "apricot", "banana", "plum")
print(grep("[aeiou]", c("sky", "apple", "plum")))
print(grep("z", words))

Example

fruits <- c("pear", "peach", "plum", "melon")
print(grep("^pe", fruits, value = TRUE))
print(grepl("l", fruits))

Key points

  • A pattern is a piece of text to look for inside each string.
  • grep gives back the positions of the strings that match.
  • The value = TRUE form of grep gives back the matching strings.
  • grepl gives back TRUE or FALSE for each string in the vector.
  • The ^ mark anchors a pattern to the start of a string.
  • The $ mark anchors a pattern to the end of a string.
  • A pattern in square brackets matches any one character from the set.

In this example the anchor ^pe picks out the words that start with pe. grepl then reports which words hold the letter l.

Note

A grep with no match gives back integer(0).

Try it Yourself

Edit the code, then press Run.

Running is simulated · rdrr.io R Snippets

Exercise

What does grep give back when no string in the vector matches the pattern?