Repetition

suppressPackageStartupMessages(library("tidyverse"))

package 㤼㸱tidyverse㤼㸲 was built under R version 3.6.3

1 Describe the equivalents of ?, +, * in {m,n} form.

Pattern {m,n} Meaning ? {0,1} Match at most 1 + {1,} Match 1 or more * {0,} Match 0 or more

Pattern	{m,n}	Meaning
?	{0,1}	Match at most 1
+	{1,}	Match 1 or more
*	{0,}	Match 0 or more

For example, let’s repeat the examples in the lecture, replacing ? with{0,1},+with{1,}, and*with{0,}`.

x <- "1888 is the longest year in Roman numerals: MDCCCLXXXVIII"
str_view(x, "CC?")

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  print.html           tools:rstudio
  print.shiny.tag      tools:rstudio
  print.shiny.tag.list tools:rstudio
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  print.htmlwidget tools:rstudio

str_view(x, "CC{0,1}")

str_view(x, "CC+")

str_view(x, "CC{1,}")

str_view_all(x, "C[LX]+")

str_view_all(x, "C[LX]{0,1}")

The lecture does not contain an example of *. This pattern looks for a “C” optionally followed by any number of “L” or “X” characters.

str_view_all(x, "C[LX]*")

str_view_all(x, "C[LX]{0,}")

2. Describe in words what these regular expressions match: (read carefully to see if I’m using a regular expression or a string that defines a regular expression.)

1. ^.*$
2. "\\{.+\\}"
3. \d{4}-\d{2}-\d{2}
4. "\\\\{4}"

The answer to each part follows.

1. ^.*$ will match any string. For example: ^.*$: c("dog", "$1.23", "lorem ipsum").

2. "\\{.+\\}" will match any string with curly braces surrounding at least one character. For example: "\\{.+\\}": c("{a}", "{abc}").

3. \d{4}-\d{2}-\d{2} will match four digits followed by a hyphen, followed by two digits followed by a hyphen, followed by another two digits. This is a regular expression that can match dates formatted like “YYYY-MM-DD” (“%Y-%m-%d”). For example: \d{4}-\d{2}-\d{2}: 2018-01-11

4. "\\\\{4}" is \\{4}, which will match four backslashes. For example: "\\\\{4}": "\\\\\\\\".

3. Create regular expressions to find all words that:

Start with three consonants. Have three or more vowels in a row. Have two or more vowel-consonant pairs in a row. The answer to each part follows.

1. Start with three consonants.

str_view(words, "^[^aeiou]{3}", match = TRUE)

2. Have three or more vowels in a row.

str_view(words, "[aeiou]{3,}", match = TRUE)

3. Have two or more vowel-consonant pairs in a row.

str_view(words, "([aeiou][^aeiou]){2,}", match = TRUE)

4. Solve the beginner regexp crosswords at https://regexcrossword.com/challenges/

Visit https://regexcrossword.com/challenges/. It’s fun.