Example 1: Base R with sub

Suppose you have a string and you want to extract the text between two characters, say [ and ].

# Sample string
text <- "Extract this [text] from the string."

# Using sub to extract text between square brackets
result <- sub(".*\\[(.*?)\\].*", "\\1", text)

# Print the result
print(result)

[1] "text"

Example 2: Base R with gsub

Now, let’s extract text between parentheses ( and ).

# Example string
text2 <- "This is a sample (extract this part) string."

# Extract string between parentheses using base R
extracted_base <- gsub(".*\\((.*)\\).*", "\\1", text2)
print(extracted_base)

[1] "extract this part"

In these examples, sub and gsub use regular expressions to find the text between the specified characters and replace the entire string with the extracted part. The pattern .*\\[(.*?)\\].* and .*\\((.*)\\).* break down as follows: - .* matches any character (except for line terminators) zero or more times. - \\[ matches the literal [ and \\( matches the literal (. - (.*?) and (.*) are non-greedy matches for any character (.) zero or more times. - \\] matches the literal ] and \\) matches the literal ). - \\1 in the replacement string refers to the first capture group, i.e., the text between [ ] and ( ).

Example 1: Using stringr::str_extract

# Load the stringr package
library(stringr)

# Using str_extract to extract text between square brackets
result_str_extract <- str_extract(text, "(?<=\\[).*?(?=\\])")

# Print the result
print(result_str_extract)

[1] "text"

Example 2: Using stringr to extract text between parentheses

# Example using stringr
extracted_str <- str_extract(text2, "\\(.*?\\)")
extracted_str <- str_sub(extracted_str, 2, -2)
print(extracted_str)

[1] "extract this part"

The str_extract function extracts the first substring matching a regex pattern. Here, (?<=\\[).*?(?=\\]) and \\(.*?\\) use lookbehind (?<=\\[) and lookahead (?=\\]) assertions to match text between [ and ], and simple matching for text between ( and ). str_sub is then used to remove the enclosing parentheses.

Example 1: Using stringi::stri_extract

# Load the stringi package
library(stringi)

# Using stri_extract to extract text between square brackets
result_stri_extract <- stri_extract(text, regex = "(?<=\\[).*?(?=\\])")

# Print the result
print(result_stri_extract)

[1] "text"

Example 2: Using stringi to extract text between parentheses

# Example using stringi
extracted_stri <- stringi::stri_extract_first_regex(text2, "\\(.*?\\)")
extracted_stri <- stringi::stri_sub(extracted_stri, 2, -2)
print(extracted_stri)

[1] "extract this part"

The stri_extract function from stringi works similarly to str_extract, utilizing regex patterns for text extraction. It’s highly optimized for performance, especially with large datasets. stri_sub is used to remove the enclosing parentheses.

Bonus: Combining Methods

For more complex scenarios, you might need to combine different methods. Here’s a quick example of how you can handle multiple extractions.

# Sample string with multiple patterns
text_multiple <- "Here is [text1] and here is (text2)."

# Using gregexpr and regmatches to extract all matches
matches <- regmatches(
  text_multiple, 
  gregexpr("(?<=\\[).*?(?=\\])|(?<=\\().*?(?=\\))", 
           text_multiple, 
           perl = TRUE)
  )

# Print the matches
print(unlist(matches))

[1] "text1" "text2"

This example uses gregexpr to find all matches and regmatches to extract them.

Until next time, keep exploring and enjoying the power of R!