String Matching in R Programming

String Matching in detail

String matching is a fundamental operation in any programming language. It is useful for locating, modifying, and removing specific substrings within a larger text. In R, string matching can be performed using direct string comparison or by leveraging regular expressions.

Regular expressions are powerful tools that contain a mix of standard characters and special symbols to define search patterns. These expressions enable efficient text extraction and pattern recognition within data.

Operations on String Matching

1. Finding a String

To locate a specific pattern within a string, R provides several functions. If the goal is to find the position of a match, the grep() function is useful. On the other hand, if we only need to verify the presence of a pattern without its exact position, the grepl() function is preferred.

grep() Function: The grep() function identifies the indices where the pattern occurs in a vector. If the pattern appears multiple times, it returns a list of all corresponding indices.

Syntax:

grep(pattern, text_vector, ignore.case=FALSE)

Parameters:

• pattern: A regular expression pattern to search for.
• text_vector: The character vector where the search is conducted.
• ignore.case: Boolean indicating whether to ignore case sensitivity (default: FALSE).

Example 1: Searching for occurrences of ‘ab’ in a character vector

words <- c("Abstract", "banana", "cab", "Abbey")
grep('ab', words)

Output:

[1] 3

Since ‘ab’ is case-sensitive by default, it does not match ‘Abstract’ and ‘Abbey’.

Example 2: Ignoring case sensitivity

words <- c("Abstract", "banana", "cab", "Abbey")
grep('ab', words, ignore.case=TRUE)

Output:

[1] 1 3 4

grepl() Function: The grepl() function returns a logical vector indicating whether the pattern exists (TRUE) or not (FALSE) in each element of the character vector.

Syntax:

grepl(pattern, text_vector, ignore.case=FALSE)

Example: Checking for the presence of ‘xy’

words <- c("oxygen", "Xylophone", "piano", "guitar")
grepl('xy', words, ignore.case=TRUE)

Output:

[1] TRUE  TRUE FALSE FALSE

2. Searching with regexpr()

The regexpr() function searches each element of the vector and returns the starting position of the match. If no match is found, it returns -1.

Syntax:

regexpr(pattern, text_vector, ignore.case=FALSE)

Example: Finding occurrences of words starting with ‘p’

words <- c("parrot", "Elephant", "penguin", "apple")
regexpr('^p', words, ignore.case=TRUE)

Output:

[1]  1 -1  1 -1

3. Finding and Replacing Strings

To replace specific occurrences of a substring, R provides the sub() and gsub() functions:

Syntax:

sub(pattern, replacement, text_vector)
gsub(pattern, replacement, text_vector)

• sub() replaces only the first occurrence of a match.
• gsub() replaces all occurrences of a match.

Example 1: Replacing the first occurrence of ‘is’ with ‘was’

sentence <- "This is a simple example. It is useful."
sub("is", "was", sentence)

Output:

[1] "Thwas is a simple example. It is useful."

4. Finding and Removing Strings

To remove specific substrings, we can use str_remove() (removes the first occurrence) and str_remove_all() (removes all occurrences).

Syntax:

str_remove(text_vector, pattern)
str_remove_all(text_vector, pattern)

Example 1: Removing the first occurrence of digits

library(stringr)
numbers <- c("123apple", "banana42", "cherry007")
str_remove(numbers, '\\d+')

Output:

[1] "apple"  "banana42" "cherry007"