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by Graham Williams
Duck Duck Go


Regexp Pattern Matching

20180608 One of the most powerful string processing concepts is the concept of regular expressions. A regular expression is a sequence of characters that describe a pattern. The concept was formalised by the American mathematician Stephen Cole Kleene. A regular expression pattern can contain a combination of alphanumeric and special characters. It is a complex topic and we take an introductory look at it here to craft regular expressions in R. An important concept is that of metacharacters which have special meaning within a regular expression. Unlike other characters that are used to match themselves, metacharacters have a specific meaning beyond the character they represent. The following table contains a list of common metacharacters used in regular expressions.

  Metacharacter Description
1 ^ Matches at the start of the string
2 $ Matches at the end of the string
3 () Define a subexpression to be matched and retrieved later.
4 $\vert$ Matches the pattern before or pattern after
5 [ ] Matches a single character that is contained within bracket
6 . Matches any single character

Such metacharacters are used to match different patterns which can be found using
base::grep(). According to gnu.org/software/grep g/re/p is a command from the command line tool ed to get the regular expression and print it.
s <- c("hands", "data", "on", "data$cience", "handsondata$cience", "handson")
grep(pattern="^data", s, value=TRUE)
## [1] "data"        "data$cience"
grep(pattern="on$", s, value=TRUE)
## [1] "on"      "handson"
grep(pattern="(nd)..(nd)", s, value=TRUE)
## [1] "handsondata$cience"

In order to match a metacharacter in R we need to escap it with $\backslash\backslash$ (double backslash).

grep(pattern="\\$", s, value=TRUE)
## [1] "data$cience"        "handsondata$cience"

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