Regular expressions

Sequence of (meta) characters.

used for pattern matching or string matching

Uses:

Data extraction
Cleaning
Data analysis
Data validation
Text mining
parsing

[abc]	a,b,c
[^abc]	any character except a, b, c
[a- z]	a to z
[A - Z]	A to Z
[a -z A- Z]	a to z, A to Z
[0 - 9]	0 to 9
[ ]?	occurs 0 or 1 time
[ ]+	Occurs 1 or more times
[ ] *	occurs 0 or more times
[ ]{n}	occurs n times
[ ]{n, }	occurs n or more times
[ ]{y, z}	occurs at least y times but less than z times
`[:alnum:]`	any alphanumeric character
`[:digit:]`	any numeric digit
`[:alpha:]`	any letter (upper or lowercase)
`[:upper:]`	any uppercase letter
`[:lower:]`	any lowercase letter

Regex Metacharacters

\d	[0 - 9]
\D	[^0 - 9]
\w	[a - z A - z 0 -9]
\W	[^\w]
`"\\s"`	a single space
`^`	Anchors the pattern to the beginning of a string.
`$`	Anchors the pattern to the end of a string.
*	Any character that is matched zero or more times

`grepl()`

Searches for a pattern within a character vector or list of character strings.

Stands for "global regular expression pattern matching with logical return."

Returns logical vector indicating whether a match was found for the pattern.

Syntax:

grepl(pattern, x, ignore.case = FALSE, perl = FALSE, fixed = FALSE, useBytes = FALSE)

pattern regular expression pattern you want to search

x the character vector or string which you want to search for the pattern

ignore.case ` optional legacy argument that specifies whether the pattern matching should be case_insensitive (TRUE)

The \1 in the replacement argument of sub() gets set to the string that is captured by the regular expression [0-9]+.

Example

`grep()`

Returns a numeric vector of indices (positions) where the pattern is found in the input vector. It returns the position of the elements that match the pattern.

Syntax

grep(pattern, x, ignore.case = FALSE, perl = FALSE, fixed = FALSE, useBytes = FALSE)

`sub()`

The function is used for pattern substitution within character strings.

It replaces the first occurrence of a specified pattern (regular expression) in a character vector with a replacement string and returns the modified character vector.

syntax

sub(pattern, replacement, x, ignore.case = FALSE, perl = FALSE, fixed = FALSE)

pattern regular expression pattern you want to search

replacement This is the string that will replace the first occurrence of the pattern in each element ox x

x the character vector or string which you want to search for the pattern

ignore.case ` optional legacy argument that specifies whether the pattern matching should be case_insensitive (TRUE)

perl: An optional logical argument that indicates whether the pattern should be treated as a Perl-compatible regular expression (TRUE) or a basic regular expression (FALSE, the default).
fixed: An optional logical argument that specifies whether pattern should be treated as a fixed string (TRUE) or as a regular expression (FALSE, the default).

Note: If you want to replace all occurrences, you can use the gsub() function.

regexpr()

is used to find the starting position of a specified pattern (regular expression) within a character vector or a list of character strings.

If no match is found, it returns -1.