R regexpr Function


regexpr returns an integer vector of the same length as text giving the starting position of the first match or -1 if there is none, with attribute "match.length", an integer vector giving the length of the matched text (or -1 for no match). The match positions and lengths are in characters unless useBytes = TRUE is used, when they are in bytes. 


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


• pattern: regular expression, or string for fixed=TRUE
• text: string, the character vector
• ignore.case: case sensitive or not
• perl: logical. Should perl-compatible regexps be used? Has priority over extended
• fixed: logical. If TRUE, pattern is a string to be matched as is. Overrides all conflicting arguments
• useBytes: logical. If TRUE the matching is done byte-by-byte rather than character-by-character


> x <- "line 4322: He is now 25 years old, and weights 130lbs"
> y <- regexpr("\\d+",x)
> y


[1] 6
attr(,"match.length")
[1] 4
attr(,"useBytes")
[1] TRUE



> x <- "line 4322: He is now 25 years old, and weights 130lbs"
> y <- regexpr("[[:digit:]]",x)
> y


[1] 6
attr(,"match.length")
[1] 1
attr(,"useBytes")
[1] TRUE



> if (y[[1]][1] != -1) print("match")


[1] "match"


Vector match:

>str <- c("Regular", "expression", "examples of R language")
>x <- regexpr("x*ress",str)
>x

[1] -1 4 -1 






Regular Expression Syntax:




Syntax
Description


\\d
Digit, 0,1,2 ... 9


\\D
Not Digit


\\s
Space


\\S
Not Space


\\w
Word


\\W
Not Word


\\t
Tab


\\n
New line


^
Beginning of the string


$
End of the string


\
Escape special characters, e.g. \\ is "\", \+ is "+"


|
Alternation match. e.g. /(e|d)n/ matches "en" and "dn"


•
Any character, except \n or line terminator


[ab]
a or b


[^ab]
Any character except a and b


[0-9]
All Digit


[A-Z]
All uppercase A to Z letters


[a-z]
All lowercase a to z letters


[A-z]
All Uppercase and lowercase a to z letters


i+
i at least one time


i*
i zero or more times


i?
i zero or 1 time


i{n}
i occurs n times in sequence


i{n1,n2}
i occurs n1 - n2 times in sequence


i{n1,n2}?
non greedy match, see above example


i{n,}
i occures >= n times



[:alnum:]
Alphanumeric characters: [:alpha:] and [:digit:]


[:alpha:]
Alphabetic characters: [:lower:] and [:upper:]


[:blank:]
Blank characters: e.g. space, tab


[:cntrl:]
Control characters


[:digit:]
Digits: 0 1 2 3 4 5 6 7 8 9


[:graph:]
Graphical characters: [:alnum:] and [:punct:]


[:lower:]
Lower-case letters in the current locale


[:print:]
Printable characters: [:alnum:], [:punct:] and space


[:punct:]
Punctuation character: ! " # $ % & ' ( ) * + , - . / : ; < = > ? @ [ \ ] ^ _ ` { | } ~


[:space:]
Space characters: tab, newline, vertical tab, form feed, carriage return, space


[:upper:]
Upper-case letters in the current locale


[:xdigit:]
Hexadecimal digits: 0 1 2 3 4 5 6 7 8 9 A B C D E F a b c d e f








					 
























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