How to Extract Strings Between Specific Characters in R


Steven P. Sanderson II, MPH


June 25, 2024


Hello, R enthusiasts! Today, we’re jumping into a common text processing task: extracting strings between specific characters. This is a great skill for data cleaning and manipulation, especially when working with raw text data. I’m going to show you how to achieve this using base R, the stringr package, and the stringi package. Let’s go!

Extracting Strings Using Base R

Base R provides several ways to extract substrings, including sub and gregexpr. Here, we’ll use sub and gsub for some examples.

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
[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)
[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 ( ).

Extracting Strings Using stringr

The stringr package, part of the tidyverse, makes string manipulation more straightforward with consistent functions.

Example 1: Using stringr::str_extract

# Load the stringr package

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

# Print the result
[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)
[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.

Extracting Strings Using stringi

The stringi package provides robust and efficient tools for string manipulation.

Example 1: Using stringi::stri_extract

# Load the stringi package

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

# Print the result
[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)
[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.

Your Turn!

Experimenting with these functions and patterns on your own datasets will help you understand their nuances. Here are a few additional exercises to solidify your understanding:

  • Extract text between parentheses ( and ).
  • Extract text between the first and last occurrences of a specific character in a string.
  • Extract all occurrences of text between two characters in a string.

Feel free to use the examples provided as a template for your own tasks.

Happy coding!

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(
           perl = TRUE)

# Print the 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!