2024-01-11
A regular expression … is a sequence of characters that define a search pattern. Usually such patterns are used by string searching algorithms for “find” or “find and replace” operations on strings, or for input validation. It is a technique developed in theoretical computer science and formal language theory.
Just to scratch the surface, here are a few special characters that cannot be directly coded. Therefore, they are escaped with a backslash, \.
\': single quote.\": double quote.\n: newline.\r: carriage return.\t: tab character.Quantifiers specify how many repetitions of the pattern.
*: matches at least 0 times.+: matches at least 1 times.?: matches at most 1 times.{n}: matches exactly n times.{n,}: matches at least n times.{n,m}: matches between n and m times.strings <- c("a", "ab", "acb", "accb", "acccb", "accccb")
grep("ac*b", strings, value = TRUE)
grep("ac*b", strings, value = FALSE)
grep("ac+b", strings, value = TRUE)
grep("ac?b", strings, value = TRUE)
grep("ac{2}b", strings, value = TRUE)
grep("ac{2,}b", strings, value = TRUE)
grep("ac{2,3}b", strings, value = TRUE)grep() stands for “global regular expression print”. grep() returns a character vector containing the selected elements, grepl() returns a logical vector of TRUE/FALSE for whether or not there was a match.
[1] "ab" "acb" "accb" "acccb" "accccb"[1] 2 3 4 5 6[1] "acb" "accb" "acccb" "accccb"[1] "ab" "acb"[1] "accb"[1] "accb" "acccb" "accccb"[1] "accb" "acccb"grep() stands for “global regular expression print”. grep() returns a character vector containing the selected elements, grepl() returns a logical vector of TRUE/FALSE for whether or not there was a match.
^: matches the start of the string.$: matches the end of the string.\b: matches the boundary of a word. Don’t confuse it with ^ $ which marks the edge of a string.\B: matches the empty string provided it is not at an edge of a word..: matches any single character,[...]: a character list, matches any one of the characters inside the square brackets. A - inside the brackets specifies a range of characters.[^...]: an inverted character list, similar to [...], but matches any characters except those inside the square brackets.\: suppress the special meaning of metacharacters in regular expression, i.e. $ * + . ? [ ] ^ { } | ( ) \. Since \ itself needs to be escaped in R, we need to escape metacharacters with double backslash like \\$.|: an “or” operator, matches patterns on either side of the |.(...): grouping in regular expressions. This allows you to retrieve the bits that matched various parts of your regular expression so you can alter them or use them for building up a new string.strings <- c("^ab", "ab", "abc", "abd", "abe", "ab 12", "a|b")
grep("ab.", strings, value = TRUE)
grep("ab[c-e]", strings, value = TRUE)
grep("ab[^c]", strings, value = TRUE)
grep("^ab", strings, value = TRUE)
grep("\\^ab", strings, value = TRUE)
grep("abc|abd", strings, value = TRUE)
grep("a[b|c]", strings, value = TRUE)
str_extract(strings, "a[b|c]")[1] "abc" "abd" "abe" "ab 12"[1] "abc" "abd" "abe"[1] "abd" "abe" "ab 12"[1] "ab" "abc" "abd" "abe" "ab 12"[1] "^ab"[1] "abc" "abd"[1] "^ab" "ab" "abc" "abd" "abe" "ab 12" "a|b" [1] "ab" "ab" "ab" "ab" "ab" "ab" "a|"Character classes allow specifying entire classes of characters, such as numbers, letters, etc. There are two flavors of character classes, one uses [: and :] around a predefined name inside square brackets and the other uses \ and a special character. They are sometimes interchangeable.
[:digit:] or \d: digits, 0 1 2 3 4 5 6 7 8 9, equivalent to [0-9].\D: non-digits, equivalent to [^0-9].[:lower:]: lower-case letters, equivalent to [a-z].[:upper:]: upper-case letters, equivalent to [A-Z].[:alpha:]: alphabetic characters, equivalent to [[:lower:][:upper:]] or [A-z].[:alnum:]: alphanumeric characters, equivalent to [[:alpha:][:digit:]] or [A-z0-9].\w: word characters, equivalent to [[:alnum:]_] or [A-z0-9_] (letter, number, or underscore).\W: not word, equivalent to [^A-z0-9_].[:blank:]: blank characters, i.e. space and tab.[:space:]: space characters: tab, newline, vertical tab, form feed, carriage return, space.\s: space, \S: not space.[:punct:]: punctuation characters, ! ” # $ % & ’ ( ) * + , - . / : ; < = > ? @ [ ] ^ _ ` { | } ~.[:graph:]: graphical (human readable) characters: equivalent to [[:alnum:][:punct:]].[:print:]: printable characters, equivalent to [[:alnum:][:punct:]\\s].More examples for practice!
meter in “The cemetery is 1 meter from the stop sign.” Also match Meter in “The cemetery is 1 Meter from the stop sign.”meter in “The cemetery is 1 meter from the stop sign.” Also match Meter in “The cemetery is 1 Meter from the stop sign.”^(1[012]|[1-9]):[0-5][0-9] (am|pm)$
The “or” operator, | has the lowest precedence and parentheses have the highest precedence, which means that parentheses get evaluated before “or”.
\bMary|Jane|Sue\b and \b(Mary|Jane|Sue)\b?The “or” operator, | has the lowest precedence and parentheses have the highest precedence, which means that parentheses get evaluated before “or”.
\bMary|Jane|Sue\b and \b(Mary|Jane|Sue)\b?string <- c("Mary", "Mar", "Janet", "jane", "Susan", "Sue")
str_extract(string, "\\bMary|Jane|Sue\\b")[1] "Mary" NA "Jane" NA NA "Sue" [1] "Mary" NA NA NA NA "Sue"