Regex In R Cheat Sheet

Posted : admin On 1/1/2022

Regular expressions are a concise and flexible tool for describing patterns in strings. This vignette describes the key features of stringr’s regular expressions, as implemented by stringi. It is not a tutorial, so if you’re unfamiliar regular expressions, I’d recommend starting at http://r4ds.had.co.nz/strings.html. If you want to master the details, I’d recommend reading the classic Mastering Regular Expressions by Jeffrey E. F. Friedl.

Regex(pattern, ignorecase = FALSE, multiline = FALSE, comments = FALSE, dotall = FALSE.) Modifies a regex to ignore cases, match end of lines as well of end of strings, allow R comments within regex's, and/or to have. Match everything including n. Strdetect('I', regex('i', TRUE)) fixed Matches raw bytes but will miss some. Regex Cheat Sheet. GitHub Gist: instantly share code, notes, and snippets.

Regular expressions are the default pattern engine in stringr. That means when you use a pattern matching function with a bare string, it’s equivalent to wrapping it in a call to regex():

You will need to use regex() explicitly if you want to override the default options, as you’ll see in examples below.

Basic matches

The simplest patterns match exact strings:

You can perform a case-insensitive match using ignore_case = TRUE:

The next step up in complexity is ., which matches any character except a newline:

You can allow . to match everything, including n, by setting dotall = TRUE:

Escaping

If “.” matches any character, how do you match a literal “.”? You need to use an “escape” to tell the regular expression you want to match it exactly, not use its special behaviour. Like strings, regexps use the backslash, , to escape special behaviour. So to match an ., you need the regexp .. Unfortunately this creates a problem. We use strings to represent regular expressions, and is also used as an escape symbol in strings. So to create the regular expression . we need the string '.'.

If is used as an escape character in regular expressions, how do you match a literal ? Well you need to escape it, creating the regular expression . To create that regular expression, you need to use a string, which also needs to escape . That means to match a literal you need to write ' — you need four backslashes to match one!

In this vignette, I use . to denote the regular expression, and '.' to denote the string that represents the regular expression.

An alternative quoting mechanism is Q...E: all the characters in ... are treated as exact matches. This is useful if you want to exactly match user input as part of a regular expression.

Special characters

Escapes also allow you to specify individual characters that are otherwise hard to type. You can specify individual unicode characters in five ways, either as a variable number of hex digits (four is most common), or by name:

  • xhh: 2 hex digits.

  • x{hhhh}: 1-6 hex digits.

  • uhhhh: 4 hex digits.

  • Uhhhhhhhh: 8 hex digits.

  • N{name}, e.g. N{grinning face} matches the basic smiling emoji.

Similarly, you can specify many common control characters:

  • a: bell.

  • cX: match a control-X character.

  • e: escape (u001B).

  • f: form feed (u000C).

  • n: line feed (u000A).

  • r: carriage return (u000D).

  • t: horizontal tabulation (u0009).

  • 0ooo match an octal character. ‘ooo’ is from one to three octal digits, from 000 to 0377. The leading zero is required.

(Many of these are only of historical interest and are only included here for the sake of completeness.)

Matching multiple characters

There are a number of patterns that match more than one character. You’ve already seen ., which matches any character (except a newline). A closely related operator is X, which matches a grapheme cluster, a set of individual elements that form a single symbol. For example, one way of representing “á” is as the letter “a” plus an accent: . will match the component “a”, while X will match the complete symbol:

There are five other escaped pairs that match narrower classes of characters:

  • d: matches any digit. The complement, D, matches any character that is not a decimal digit.

    Technically, d includes any character in the Unicode Category of Nd (“Number, Decimal Digit”), which also includes numeric symbols from other languages:

  • s: matches any whitespace. This includes tabs, newlines, form feeds, and any character in the Unicode Z Category (which includes a variety of space characters and other separators.). The complement, S, matches any non-whitespace character.

  • p{property name} matches any character with specific unicode property, like p{Uppercase} or p{Diacritic}. The complement, P{property name}, matches all characters without the property. A complete list of unicode properties can be found at http://www.unicode.org/reports/tr44/#Property_Index.

  • w matches any “word” character, which includes alphabetic characters, marks and decimal numbers. The complement, W, matches any non-word character.

    Technically, w also matches connector punctuation, u200c (zero width connector), and u200d (zero width joiner), but these are rarely seen in the wild.

  • b matches word boundaries, the transition between word and non-word characters. B matches the opposite: boundaries that have either both word or non-word characters on either side.

You can also create your own character classes using []:

  • [abc]: matches a, b, or c.
  • [a-z]: matches every character between a and z (in Unicode code point order).
  • [^abc]: matches anything except a, b, or c.
  • [^-]: matches ^ or -.

There are a number of pre-built classes that you can use inside []:

  • [:punct:]: punctuation.
  • [:alpha:]: letters.
  • [:lower:]: lowercase letters.
  • [:upper:]: upperclass letters.
  • [:digit:]: digits.
  • [:xdigit:]: hex digits.
  • [:alnum:]: letters and numbers.
  • [:cntrl:]: control characters.
  • [:graph:]: letters, numbers, and punctuation.
  • [:print:]: letters, numbers, punctuation, and whitespace.
  • [:space:]: space characters (basically equivalent to s).
  • [:blank:]: space and tab.

These all go inside the [] for character classes, i.e. [[:digit:]AX] matches all digits, A, and X.

You can also using Unicode properties, like [p{Letter}], and various set operations, like [p{Letter}--p{script=latin}]. See ?'stringi-search-charclass' for details.

Alternation

is the alternation operator, which will pick between one or more possible matches. For example, abc def will match abc or def.

Note that the precedence for is low, so that abc def matches abc or def not abcyz or abxyz.

Grouping

You can use parentheses to override the default precedence rules:

Parenthesis also define “groups” that you can refer to with backreferences, like 1, 2 etc, and can be extracted with str_match(). For example, the following regular expression finds all fruits that have a repeated pair of letters:

You can use (?:...), the non-grouping parentheses, to control precedence but not capture the match in a group. This is slightly more efficient than capturing parentheses.

This is most useful for more complex cases where you need to capture matches and control precedence independently.

Anchors

By default, regular expressions will match any part of a string. It’s often useful to anchor the regular expression so that it matches from the start or end of the string:

  • ^ matches the start of string.
  • $ matches the end of the string.

To match a literal “$” or “^”, you need to escape them, $, and ^.

For multiline strings, you can use regex(multiline = TRUE). This changes the behaviour of ^ and $, and introduces three new operators:

  • ^ now matches the start of each line.

  • $ now matches the end of each line.

  • A matches the start of the input.

  • z matches the end of the input.

  • Z matches the end of the input, but before the final line terminator, if it exists.

Repetition

You can control how many times a pattern matches with the repetition operators:

  • ?: 0 or 1.
  • +: 1 or more.
  • *: 0 or more.

Note that the precedence of these operators is high, so you can write: colou?r to match either American or British spellings. That means most uses will need parentheses, like bana(na)+.

You can also specify the number of matches precisely:

  • {n}: exactly n
  • {n,}: n or more
  • {n,m}: between n and m

By default these matches are “greedy”: they will match the longest string possible. You can make them “lazy”, matching the shortest string possible by putting a ? after them:

  • ??: 0 or 1, prefer 0.
  • +?: 1 or more, match as few times as possible.
  • *?: 0 or more, match as few times as possible.
  • {n,}?: n or more, match as few times as possible.
  • {n,m}?: between n and m, , match as few times as possible, but at least n.

You can also make the matches possessive by putting a + after them, which means that if later parts of the match fail, the repetition will not be re-tried with a smaller number of characters. This is an advanced feature used to improve performance in worst-case scenarios (called “catastrophic backtracking”).

  • ?+: 0 or 1, possessive.
  • ++: 1 or more, possessive.
  • *+: 0 or more, possessive.
  • {n}+: exactly n, possessive.
  • {n,}+: n or more, possessive.
  • {n,m}+: between n and m, possessive.

A related concept is the atomic-match parenthesis, (?>...). If a later match fails and the engine needs to back-track, an atomic match is kept as is: it succeeds or fails as a whole. Compare the following two regular expressions:

The atomic match fails because it matches A, and then the next character is a C so it fails. The regular match succeeds because it matches A, but then C doesn’t match, so it back-tracks and tries B instead.

Look arounds

These assertions look ahead or behind the current match without “consuming” any characters (i.e. changing the input position).

  • (?=...): positive look-ahead assertion. Matches if ... matches at the current input.

  • (?!...): negative look-ahead assertion. Matches if ...does not match at the current input.

  • (?<=...): positive look-behind assertion. Matches if ... matches text preceding the current position, with the last character of the match being the character just before the current position. Length must be bounded
    (i.e. no * or +).

  • (?<!...): negative look-behind assertion. Matches if ...does not match text preceding the current position. Length must be bounded
    (i.e. no * or +).

These are useful when you want to check that a pattern exists, but you don’t want to include it in the result:

Short for regular expression, regex is a handy way to create patterns that help match, find, and manage text. When you want to learn regex, it's best to start simply, and expand your knowledge as you find yourself needing more powerful expressions.

At first, regex examples will seem like a foreign language. Just looking at a regular expressions cheat sheet won't help; you first have to understand where to use regex and why you want to use it.

Get your own regex library

Get an Expressions app and improve Regex performance. Best utilities in one pack, give it a go!

We'll provide you with a beginner's regex tutorial, a handy regex cheat sheet, and tell you about some apps to help you along the way.

What Does Regex Do?

The purpose of regex is not to code full programs. Instead, it's a method to get data from massive bodies of text. It's useful for many of the most popular programming languages today, like Java, JavaScript, C-based languages, Perl, Python, Delphi, Ruby, R, and many more.

Regex In R Cheat Sheet Download

If you've used HTML before, it's probably fair to say a regex expression is a lot like markup. It uses anchors, quantifiers, operators, classes, and flags to help you parse what's in the text you're asking it to search.

A regex expression is really trying to find what you've asked it to search for. When there's a regex match, it's verification your expression is correct. You could simply type 'set' into a Regex parser, and it would find the word 'set' in the first sentence. You could also use 's.t' in the parser, which will find all words that begin with 's' and end with 't'. If you have to deal with a massive amount of text, this is a life-saver.

Regular Expression Cheat Sheet

Getting Started with Regex

Keep in mind regex is an expression. It's meant to be used in your code as an expression, not as a coding language.

A great tool for getting started with regex is Expressions, a Mac app that gives you a standalone sandboxed environment to work with regex expressions. It has regex highlighting to show your matches, a minimalist interface, and handy reference chart at your fingertips. There's a really sharp live preview for regex matching, too.

Now let's get into the regular expression cheat sheet!

Regex Cheat Sheet

Here's a very simple cheat sheet for regex:

Anchors

  • A — Start of string
  • Z — End of string
  • b — Word boundary
  • B — Not word boundary
  • < — Start of word
  • > — End of word
  • — Matches previous OR next character
  • ^Here – Matches any string that begins with 'Here'
  • finish$ - Matches any string that ends with 'finish'
  • ^Here finish$ - Matches any string that begins with 'Here' and ends with 'finish'
  • here – Matches any string with 'here' in the string

Quanitifers

  • xyz* - Matches strings which have xy in them, but not necessarily z.
  • xyz+ - Matches strings which have xy followed by at least one z.
  • xyz? - Matches strings which have xy and either zero or one z.
  • xyz{2} - Matches strings which have xy followed by exactly two z.
  • xyz{2, } - Matches strings which have xy followed by two or more z.
  • xyz{2, 8} - Matches strings which have xy followed by at least 2 z, and up to eight z.
  • x(yz)* - Matches strings which have x followed by zero or more uses of yz.
  • X(yz){2, 8} - Matches strings which have x followed by two through 8 uses of the sequence yz.

Regex In R Cheat Sheet

As you see, our regex examples are starting to get a little more complex – almost mathematical! Now, let's get into operators, which can expand on your regex parsing quite a bit.

Operators

  • x(yz) - Matches strings where x is followed by either y or z.

With the 'or' operator, you can start to capture sequences that may be slightly off. Let's say in a body of text, you were discussing desserts. Your fingers were moving too fast, and you were typing 'dessetrs' half the time; instead of reading through it all, you could use the 'or' operator to discover your mistakes: e(rt).

The problem here is you'd also find a ton of other words. In the paragraph above, you'd get 'operator' and 'were' along with many other words. We can solve that in just a minute.

First, another regex operator:

  • x[yz] Matches strings where x is matched, but not y and z.

Using this would return a lot of matches, too. It will find everything in the aforementioned paragraph which includes 'et' or 'er' – and that includes your 'dessetrs' error word, as well as 'desserts' and other words like 'discover.'

So how can we find the error word, and block the rest out?

Character Classes

  • d - Matches a single character that is a digit.
  • w - Matches a single character that is a word character (no numbers).
  • s - Matches any whitespace character.
  • . - Matches any character.
  • t – Matches any tab.
  • r – Matches any return.

In a regex engine, you could enter 'et' and find your 'dessetrs' error word, but it would also show 'let's'. If this were a massive body of text, who knows how many times you'd find 'et' used similarly.

But you can also use character classes. If we entered 'etw' into the regex parser, it would return our error word – and only our error word! Finally, the right regex for our needs.

Tools to learn, build, and test RegEx

Regex is handy for beginners, and really useful when you start to tinker with its broad set of features and functionality.

This is why we really suggest a Regex app like Expressions. It provides a safe environment to learn regex without worrying about screwing anything up. We're also big fans of TeaCode and CodeRunner; all three make for a solid coding environment.

Regex in r cheat sheet pdf

You can speed up your coding with TeaCode, a text expander for Mac, and with plugins for IDEs like Atom, Visual Studio Code, JetBrains, and Sublime Text. It has over 80 ready-to-use shortcode expanders that blossom into code that can be compiled within your IDE.

If you're looking for a really handy lightweight IDE for Mac, CodeRunner may be just what you're looking for. It has support for over 25 languages and 230 syntax highlighters, and arrives in a familiar format with sidebars and customization options to suit anyone.


They're also available for free as part of a seven-day trial of Setapp, which is just $9.99 per month after the trial period ends. So give it a try!

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Regex In R Cheat Sheet Pdf

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