Regular Expressions!!

Size: px

Start display at page:

Download "Regular Expressions!!"

Karin Dina Johnson
6 years ago
Views:

1 Regular Expressions!!

2 In your mat219_class project 1. Copy code from D2L to download regex-prac9ce.r, and run in the Console. 2. Open a blank R script and name it regex-notes. library(tidyverse)

3 regular expressions

4 stringr patterns x <- c("abc", "a.c", "a?c") str_detect(x, paiern = "a.c") str_detect(x, paiern = reg( "a.c")) The default interpreta9on for paierns is a regular expression paiern is passed to reg()

5 stringr patterns x <- c("abc", "a.c", "a?c") str_detect(x, paiern = "a.c") str_detect(x, paiern = coll( "a.c")) The default interpreta9on for paierns is a regular expression coll() can be used for literal matches

6 Regular expressions A regular expression (regex for short) is a special text string for describing a search paiern. ^\\w+@[a-za-z_]+?\\.[a-za-z]{2,3}$

7 Main stringr functions str_detect(string, paiern) detects the presence or absence of a paiern in a string. str_locate(string, paiern) locates the first posi9on of a paiern and return a matrix with start and end. str_extract(string, paiern) extracts text corresponding to the first match. str_split(string, paiern) splits string into pieces and returns a list of character vectors. str_replace(string, paiern, replacement) replaces the first matched paiern and returns a character vector.

8 Inspecting regular expressions str_view(string, paiern) displays the first match (if any) in a string. str_view_all(string, paiern) displays all matches (if any) in a string.

9 basic matches

10 Literal matches and the special. character x <- c("apple", "banana", "pear") str_view_all(x, "an") Regexs use literal matching unless a special character or class is used str_view_all(x, a. ) The special character. matches any single character except newline

11 Escaping dot <- \\. writelines(dot) x <- c( abc", a.c", a?c ) str_view_all(x, a\\.c") Backslash ( \ ) is the escape character for regexs and for strings in R. So to literally match a special character like., we need \\.

12 Escaping backslash <- \\ writelines(backslash) x <- c( abc", a\\c", a?c ) str_view_all(x, a\\\\c") To literally match the escape character \, we need \\\\

13 anchors

14 anchors ^ $ \b \B : matches the start of a string : matches the end of a string : matches a word (A-Z, a-z, 0-9, _) boundary : matches anything but a word boundary

15 Beginning and end of string x <- c("apple", "banana", "pear") str_view_all(x, "^a") ^ means the beginning of the string str_view_all(x, "a$") $ means the end of the string

16 Beginning and end of words x <- c( early ear pear ear") str_view_all(x, \\bear\\b") use \b to avoid matching within a larger word

17 character classes

18 Character classes \s \S \d \D \w \W : matches any whitespace (space, tab, newline) : matches anything but whitespace (space, tab, newline) : matches any digit : matches anything but a digit : matches any (single character) word (A-Z, a-z, 0-9, or _) : matches anything but a (single character) word

19 classes x <- c("abc", a2c", "a?c") str_view(x, paiern = a\\wc") classes like \w work like the special character., just matching different characters str_view(x, paiern = a.c")

20 ranges

21 Ranges [abc] : matches a, b, or c [^abc] : matches anything except a, b, or c [a-q] : matches any lowercase leier between a and q [A-Q] : matches any uppercase leier between A and Q [0-7] : matches any digit between 0 and 7

22 Ranges x <- c("apple 495", "banana 327", Banana 991") str_view_all(x, [Bb]anana") str_view_all(x, [a-d0-5]")

23 quantifiers

24 Quantifiers * : 0 or more + : 1 or more? : 0 or 1 {3} : exactly 3 {3,} : 3 or more {,5} : at most 5 {3,5} : between 3 and 5

25 Repetition using quantifiers x <- "1888 is the longest year in Roman numerals: MDCCCLXXXVIII str_view(x, "CC?") str_view(x, "CC+") str_view(x, "C[LX]+")

26 Quantifiers and precedence x <- c( banana, cucumber, papaya ) str_view(x, (an)+ ) precedence of quan9fiers is high, so parentheses are needed for mul9ple characters

27 Making a quantifier lazy x <- should we stop or go you decide str_view(x, ^.+@ ) quan9fiers are greedy (match the longest string) by default str_view(x, ^.+?@ ) following a quan9fier with? makes it lazy (match the shortest string)

28 alternation

29 Alternation operator x <- c("apple", gape", "pear") str_view_all(x, ^ap p.") has low precedence str_view(x, "^(ap p.)") use () to change precedence, just like for quan9fiers

30 grouping and backreferences

$papaya ) str_view(x, (an){2} ) Suppose we would like$ to search for any pair of repeated leiers str_view(x,

31 Grouping and backreferences x <- c( banana, cucumber, papaya ) str_view(x, (an){2} ) Suppose we would like to search for any pair of repeated leiers str_view(x, (..){2} ) This finds the paiern any two characters twice

$.)\\1 ) () form a capturing group, and backreference \\1 refers to the paiern that$

32 Grouping and backreferences x <- c( banana, cucumber, papaya ) str_view(x, (..)\\1 ) () form a capturing group, and backreference \\1 refers to the paiern that was captured

33 lookarounds

34 Lookarounds (?= ) (?!...) (?<= ) (?<!...) : look ahead for : look ahead for anything but : look behind for : look behind for anything but

$Lookarounds x <- c("1 piece", "2 pieces", "3") str_view(x, "\\d+(?= pieces?$

35 Lookarounds x <- c("1 piece", "2 pieces", "3") str_view(x, "\\d+(?= pieces?)") look ahead for piece or pieces y <- c("100", "$400") str_view(y, "(?<=\\$)\\d+") look behind for $

Bioinformatics Programming. EE, NCKU Tien-Hao Chang (Darby Chang)

Bioinformatics Programming. EE, NCKU Tien-Hao Chang (Darby Chang) Bioinformatics Programming EE, NCKU Tien-Hao Chang (Darby Chang) 1 Regular Expression 2 http://rp1.monday.vip.tw1.yahoo.net/res/gdsale/st_pic/0469/st-469571-1.jpg 3 Text patterns and matches A regular