grep and grepl in R Programming

Introduction

Use grep and grepl to find if a word or pattern exists in text. They help you search inside lists or strings easily.

You want to find which words in a list contain a certain pattern.

You need to check if a sentence has a specific word.

You want to filter data rows that include a keyword.

You want to know if a pattern appears anywhere in your text.

You want to get the positions of matching words in a list.

Syntax

R Programming

grep(pattern, x, ignore.case = FALSE, value = FALSE)
grepl(pattern, x, ignore.case = FALSE)

pattern is the text or regular expression you want to find.

x is the vector of strings to search in.

ignore.case lets you ignore uppercase/lowercase differences.

value = TRUE in grep returns matching strings instead of their positions.

Examples

Find positions of words containing "apple".

R Programming

words <- c("apple", "banana", "grape", "pineapple")
grep("apple", words)

Returns TRUE or FALSE for each word if it contains "apple".

R Programming

grepl("apple", words)

Returns the actual words that contain "apple".

R Programming

grep("apple", words, value = TRUE)

Checks for "APPLE" ignoring case differences.

R Programming

grepl("APPLE", words, ignore.case = TRUE)

Sample Program

This program searches for the pattern "Cat" in a list of words. It shows the positions where "Cat" appears, a TRUE/FALSE list for each word, the matching words themselves, and a case-insensitive check.

R Programming

words <- c("Cat", "Dog", "Caterpillar", "Bird")
positions <- grep("Cat", words)
matches <- grepl("Cat", words)
cat_words <- grep("Cat", words, value = TRUE)
ignore_case <- grepl("cat", words, ignore.case = TRUE)

print(positions)
print(matches)
print(cat_words)
print(ignore_case)

Important Notes

grep returns positions by default, but you can get matching values with value = TRUE.

grepl returns a logical vector (TRUE/FALSE) for each element.

Both functions support regular expressions for advanced pattern matching.

Summary

grep finds positions or values matching a pattern.

grepl tells you TRUE or FALSE if a pattern exists in each element.

Use ignore.case = TRUE to ignore letter case differences.