Regular expressions in R in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of grepl with pattern and ignore.case

What is the output of this R code snippet?

R Programming

text <- c("Apple", "banana", "Cherry", "date")
result <- grepl("a", text, ignore.case = TRUE)
print(result)

A [1] FALSE TRUE TRUE TRUE

B [1] TRUE FALSE TRUE FALSE

C [1] TRUE TRUE FALSE TRUE

D [1] TRUE TRUE TRUE TRUE

💡 Hint

grepl returns TRUE if the pattern is found in each element. ignore.case = TRUE makes it case insensitive.

The pattern 'a' appears in 'Apple' (A), 'banana' (a), and 'date' (a) ignoring case, but not in 'Cherry'. So the result is [1] TRUE TRUE FALSE TRUE, which is option C.

❓ Predict Output

intermediate

Result of sub function with regex

What is the output of this R code?

R Programming

text <- "The rain in Spain"
result <- sub("ain", "XXX", text)
print(result)

A "The rXXX in Spain"

B "The rain in SpXXX"

C "The rain in Spain"

D "The rXXX in SpXXX"

💡 Hint

sub replaces only the first match of the pattern.

The pattern 'ain' appears twice: in 'rain' and 'Spain'. sub replaces only the first occurrence, so only 'rain' changes to 'rXXX'.

🔧 Debug

advanced

Identify the error in regex pattern

What error does this R code produce?

R Programming

text <- "abc123"
result <- grepl("[a-z{3}", text)
print(result)

A Error: invalid regular expression '[a-z{3}'

B SyntaxError: invalid regular expression

C [1] TRUE

D [1] FALSE

💡 Hint

Check if the regex pattern has balanced brackets.

The pattern '[a-z{3}' is missing a closing bracket ']', so R throws an error about invalid regex.

❓ Predict Output

advanced

Output of regmatches with gregexpr

What is the output of this R code?

R Programming

text <- "one1 two22 three333"
matches <- regmatches(text, gregexpr("\\d+", text))
print(matches)

A [[1]] "one" "two" "three"

B [[1]] "one1" "two22" "three333"

C [[1]] "1" "2" "3"

D [[1]] "1" "22" "333"

💡 Hint

gregexpr finds all matches of one or more digits, regmatches extracts them.

The pattern '\\d+' matches sequences of digits. The text has '1', '22', and '333' as digit sequences.

🧠 Conceptual

expert

Number of matches with lookahead in R regex

How many matches does this R code find?

R Programming

text <- "aaaa"
matches <- gregexpr("a(?=a)", text, perl=TRUE)
length(regmatches(text, matches)[[1]])

A 2

B 3

C 4

D 1

💡 Hint

Lookahead '(?=a)' matches 'a' only if followed by another 'a'.

In 'aaaa', the pattern matches the first three 'a's because each is followed by another 'a'. The last 'a' is not followed by 'a', so total matches are 3.