Regex Cheat Sheet

February 19, 2019

A regular expression, or ‘regex’, is used to match parts of a string. Below is my cheat sheet for creating regular expressions.

Testing a regex

Use the .test() method

let testString = "My test string"
let testRegex = /string/
testRegex.test(testString)

Testing multiple patterns

Use the OR operator (|)

const regex = /yes|no|maybe/

Ignoring case

Use the i flag for case insensitivity

const caseInsensitiveRegex = /ignore case/i
const testString = "We use the i flag to iGnOrE CasE"
caseInsensitiveRegex.test(testString) // true

Extracting the first match to a variable

Use the .match() function

const match = "Hello World!".match(/hello/i) // "Hello"

Extracting all of the matches in an array

Use the g flag

const testString = "Repeat repeat rePeAT"
const regexWithAllMatches = /Repeat/gi
testString.match(regexWithAllMatches) // ["Repeat", "repeat", "rePeAT"]

Matching any character

Use the wildcard character . to be a placeholder for any character

// To match "cat", "BAT", "fAT", "mat"
const regexWithWildcard = /.at/gi
const testString = "cat BAT cupcake fAT mat dog"
const allMatchingWords = testString.match(regexWithWildcard) // ["cat", "BAT", "fAT", "mat"]

Matching a single character with multiple possibilities

Use character classes, which allow you to define a group of characters you wish to match
You place them inside square brackets []

// Match "cat" "fat" and "mat" but not "bat"
const regexWithCharClass = /[cfm]at/g
const testString = "cat fat bat mat"
const allMatchingWords = testString.match(regexWithCharClass) // ["cat", "fat", "mat"]

Match letters of the alphabet

Use a range within the character set [a-z]

const regexWithCharRange = /[a-e]at/
const catString = "cat"
const batString = "bat"
const fatString = "fat"

regexWithCharRange.test(catString) // true
regexWithCharRange.test(batString) // true
regexWithCharRange.test(fatString) // false

Match specific numbers and letters

You can also use the hyphen to match numbers

const regexWithLetterAndNumberRange = /[a-z0-9]/gi
const testString = "Emma19382"
testString.match(regexWithLetterAndNumberRange) // true

Match a single, unknown character

To match a set of characters you don’t want to have, use the negated character set
To negate a character set, use a caret ^

const allCharsNotVowels = /[^aeiou]/gi
const allCharsNotVowelsOrNumbers = /[^aeiou0-9]/gi

Match characters that occur one or more times in a row

Use the + symbol

const oneOrMoreAsRegex = /a+/gi
const oneOrMoreSsRegex = /s+/gi
const cityInFlorida = "Tallahassee"

cityInFlorida.match(oneOrMoreAsRegex) // ['a', 'a', 'a'];
cityInFlorida.match(oneOrMoreSsRegex) // ['ss'];

Matches characters that occur zero or more times in a row

Use the asterisk *

const zeroOrMoreOsRegex = /hi*/gi
const normalHi = "hi"
const happyHi = "hiiiiii"
const twoHis = "hiihii"
const bye = "bye"

normalHi.match(zeroOrMoreOsRegex) // ["hi"]
happyHi.match(zeroOrMoreOsRegex) // ["hiiiiii"]
twoHis.match(zeroOrMoreOsRegex) // ["hii", "hii"]
bye.match(zeroOrMoreOsRegex) // null

Lazy Matching

The smallest part of a string that matches the given requirements
Regex, by default, are greedy (matches the longest portion of a string meeting the given requirements)
Use the ? character to lazy match

const testString = "catastrophe"
const greedyRexex = /c[a-z]*t/gi
const lazyRegex = /c[a-z]*?t/gi

testString.match(greedyRexex) // ["catast"]
testString.match(lazyRegex) // ["cat"]

Match starting string patterns

To test for a match of characters at the beginning of a string, use the caret ^, but outside of the character set

const emmaAtFrontOfString = "Emma likes cats a lot."
const emmaNotAtFrontOfString = "The cats Emma likes are fluffy."
const startingStringRegex = /^Emma/

startingStringRegex.test(emmaAtFrontOfString) // true
startingStringRegex.test(emmaNotAtFrontOfString) // false

Match ending string patterns

Use the dollar sign $ at the end of a regex to check whether a pattern exists at the end of a string

const emmaAtBackOfString = "The cats do not like Emma"
const emmaNotAtBackOfString = "Emma loves the cats"
const startingStringRegex = /Emma$/

startingStringRegex.test(emmaAtBackOfString) // true
startingStringRegex.test(emmaNotAtBackOfString) // false

Matching all letters and numbers

Use the \word shorthand

const longHand = /[A-Za-z0-9_]+/
const shortHand = /\w+/
const numbers = "42"
const myFavoriteColor = "magenta"

longHand.test(numbers) // true
shortHand.test(numbers) // true
longHand.test(myFavoriteColor) // true
shortHand.test(myFavoriteColor) // true

Match everything except letters & numbers

You can use for the opposite of \w with \W

const noAlphaNumericCharRegex = /\W/gi
const weirdCharacters = "!_$!!"
const alphaNumericCharacters = "ab283AD"

noAlphaNumericCharRegex.test(weirdCharacters) // true
noAlphaNumericCharRegex.test(alphaNumericCharacters) // false

Match all numbers

You can use a character set [0-9], or use the shorthand \d

const digitsRegex = /\d/g
const stringWithDigits = "My cat eats $20.00 worth of food a week."

stringWithDigits.match(digitsRegex) // ["2", "0", "0", "0"]

Match all non-numbers

You can use the opposite of \d with \D

const nonDigitsRegex = /\D/g
const stringWithLetters = "101 degrees"

stringWithLetters.match(nonDigitsRegex) // [" ", "d", "e", "g", "r", "e", "e", "s"]

Matching whitespace

Use \s to match white space and carriage returns

const sentenceWithWhitespace = "I like cats!"
var spaceRegex = /\s/g
whiteSpace.match(sentenceWithWhitespace) // [" ", " "]

Matching non-whitespace

You can use the opposite of \s with \S

const sentenceWithWhitespace = "C a t"
const nonWhiteSpaceRegex = /\S/g
sentenceWithWhitespace.match(nonWhiteSpaceRegex) // ["C", "a", "t"]

Matching character counts

You can specify a specific number of characters in a row using {lowerBound, upperBound}

const regularHi = "hi"
const mediocreHi = "hiii"
const superExcitedHey = "heeeeyyyyy!!!"
const excitedRegex = /hi{1,4}/

excitedRegex.test(regularHi) // true
excitedRegex.test(mediocreHi) // true
excitedRegex.test(superExcitedHey) //false

Matching lowest number of character counts

You can define only a minimum number of character requirements with {lowerBound,}
This is called a quantity specifier

const regularHi = "hi"
const mediocreHi = "hiii"
const superExcitedHey = "heeeeyyyyy!!!"
const excitedRegex = /hi{2,}/

excitedRegex.test(regularHi) // false
excitedRegex.test(mediocreHi) // true
excitedRegex.test(superExcitedHey) //false

Matching an exact number of character counts

You can specify the exact number of character requirements with {requiredCount}

const regularHi = "hi"
const bestHi = "hii"
const mediocreHi = "hiii"
const excitedRegex = /hi{2}/

excitedRegex.test(regularHi) // false
excitedRegex.test(bestHi) // true
excitedRegex.test(mediocreHi) //false

Matching all or none of a character

To check whether a character exists, use the ?

const britishSpelling = "colour"
const americanSpelling = "Color"
const languageRegex = /colou?r/i

languageRegex.test(britishSpelling) // true
languageRegex.test(americanSpelling) // true