Function regexpr() with R

Function regexpr()

The regexpr() function in R is used to search for patterns in character strings using regular expressions (regex). It returns the position of the first occurrence of the pattern in each string, as well as the length of that occurrence.

Syntax 

regexpr(pattern, text, ignore.case = FALSE, perl = FALSE, fixed = FALSE, useBytes = FALSE)

 Arguments:

• pattern:
  • A character string containing the pattern to search for. This is usually a regular expression.
  • Example: “\\d+” to search for numbers.
• text:
  • The character string or vector of strings to search within.
  • Example: “There are 123 apples”.
• ignore.case:
  • A boolean (TRUE or FALSE) indicating whether the search should be case-insensitive (default: FALSE).
  • Example: TRUE to ignore case when searching.
• perl:
  • A boolean (TRUE or FALSE) indicating whether to use Perl-compatible regex syntax (default: FALSE).
  • Example: TRUE to use Perl syntax.
• fixed:
  • A boolean (TRUE or FALSE) indicating whether the pattern should be treated as a fixed string rather than a regular expression (default: FALSE).
  • Example: TRUE to search for the exact string without interpreting special characters.
• useBytes:
  • A boolean (TRUE or FALSE) indicating whether to treat the pattern and text as bytes rather than characters (default: FALSE).
  • Example: TRUE to process the text as bytes.

Return Values

• The function returns a vector of the same length as text containing:
  • The position of the start of the first occurrence of the pattern (1-based index).
  • The length of that occurrence.
  • If the pattern is not found, the function returns -1.

Practical Examples

Example 1: Simple Pattern Search 

# Search for a number in a string
result <- regexpr("\\d+", "There are 123 apples")
result
# [1] 11
# attr(,"match.length")
# [1] 3

•  Position: 11 (the start of the first occurrence of “123”).
• Length: 3 (the length of the number “123”).

Example 2: Case-Insensitive Search 

# Search for "hello" case-insensitively
result <- regexpr("hello", "Hello World", ignore.case = TRUE)
result
# [1] 1
# attr(,"match.length")
# [1] 5

• Position: 1 (the start of the first occurrence of “Hello”).
• Length: 5 (the length of “Hello”).

Example 3: Fixed String Search 

#Search for a fixed string rather than a regex
result <- regexpr("123", "The number is 1234", fixed = TRUE)
result
# [1] 16
# attr(,"match.length")
# [1] 3

• Position: 16 (the start of “123”).
• Length: 3 (the length of “123”).

Example 4: Using Perl Syntax 

# Search for a pattern using Perl syntax
result <- regexpr("\\d{2,}", "There are 123 apples", perl = TRUE)
result
# [1] 11
# attr(,"match.length")
# [1] 3

•  Position: 11 (the start of the first occurrence of “123”).
• Length: 3 (the length of “123”).

Points to Note

• Start Position: The position returned is 1-based. If the pattern is not found, -1 is returned.
• Match Length: The length of the match is given as an attribute “match.length”.
• Regular Expression Options: You can adjust the behavior of the search using ignore.case, perl, fixed, and useBytes arguments to suit specific needs.
• Performance: Using fixed = TRUE can be faster for fixed strings since it does not require regex interpretation.