Regular Expressions

What are Regular Expressions? #

A type of formal language (pattern) that can be used to search and replace strings.
From simple character searches to complex character matching functions like email and password validation, regular expression patterns can perform these tasks quickly.

Role of Regular Expressions #

Character Search
Character Replacement
Character Extraction

Test Site #

https://regexr.com/

Creating JS Regular Expressions #

Constructor Function Method #

You can use it by calling the RegExp constructor function.

const regexp1 = new RegExp("abc");
// new RegExp(pattern)

const regexp2 = new RegExp("^abc","gi");
// new RegExp(pattern , flags)

Literal Method #

Regular expressions use patterns enclosed by / as literals.

const regexp1 = /^abc/;
// /pattern/

const regexp2 = /^abc/gi;
// /pattern/flags

gi : Case-insensitive

Regular Expression Methods #

Method	Syntax	Description
test	`regex.test(string)`	Match status (Boolean)
match	`string.match(regex)`	Returns an array of matching characters
replace	`string.replace(regex, replacement)`	Replaces matching characters

let str = `
010-1234-5678
s22043@gsm.hs.kr
The quick brown fox jumps over the lazy dog.
abbcccdddd
`

const regexp = /fox/gi

console.log(regexp.test(str)) // true
console.log(str.replace(regexp,'AAA')) // fox >> AAA

Flags (Options) #

Flag	Description
g	Global match
i	Case-insensitive match (English)
m	Multiline match

g, gi

let str = `
010-1234-5678
s22043@gsm.hs.kr
The quick brown fox jumps over the lazy dog.
abbcccdddd
the apple
`

const regexp = /the/
console.log(str.match(regexp)) // Matches only one word containing lowercase 'the'

const regexp2 =/the/g
console.log(str.match(regexp)) // Outputs all lowercase 'the'

const regexp3 = /the/gi
console.log(str.match(regexp)) // Outputs all 'the', including uppercase 'The'

let str = `
010-1234-5678
s22043@gsm.hs.kr
The quick brown fox jumps over the lazy dog.
abbcccdddd
the apple
`
console.log(str.match(/\.$/gim)) // ["."]

$ Returns only the first sentence that matches the regular expression.

\ (Escape character) refers to a character whose original function changes when preceded by a backslash.

Patterns (Expressions) #

Pattern	Description
^ab	Matches 'ab' at the `start` of a line
ab$	Matches 'ab' at the `end` of a line
.	Matches any single character
a	b
ab?	Matches 'ab' or 'a' (b is optional)
{3}	Matches exactly 3 consecutive occurrences
{3 ,}	Matches 3 or more consecutive occurrences
{3 , 5}	Matches 3 to 5 consecutive occurrences

let str = `
010-1234-5678
s22043@gsm.hs.kr
The quick brown fox jumps over the lazy dog.
abbcccdddd
the apple
d
https://local.1234
http://local.12345`


console.log(
  str.match(/d$/g)  
) // ["d"] 

console.log(
  str.match(/d$/gm)  
) // ["d"] ["d"] 

console.log(
  str.match(/^t/gim)
) // ["t"] , ["T"]

console.log(
  str.match(/./g) // An array of all characters is returned.
)

console.log(
  str.match(/.a..e/g)
) // ["apple"]

console.log(
  str.match(/fox|dog/g)
) // ["fox"] , ["dog"] if 'g' is absent, only 'fox' is output

console.log(
  str.match(/https?/g)
) // ["https"] ,["http"]

console.log(
  str.match(/d{2}/g)
) // dd dd << is an array

console.log(
  str.match(/\b\w{2,3}\b/g)
  // Outputs only words consisting of 2 to 3 characters, regardless of English letters or numbers
)

Pattern	Description
[abc]	a or b or c
[a-z]	Matches characters in the range from a to z (lowercase English letters)
[A-Z]	Matches characters in the range from A to Z (uppercase English letters)
[0-9]	Matches characters in the range from 0 to 9 (numbers)
[가-힣]	Matches characters in the range from '가' to '힣' (Korean characters)
\w	Matches 63 characters (Word, 52 uppercase/lowercase English letters + 10 numbers + _)
\b	Word boundary (matches positions that are not part of a 63-character word)
\d	Matches a digit
\s	Matches whitespace
(?=)	Positive lookahead
(?<=)	Positive lookbehind

let str = `
010-1234-5678
s22043@gsm.hs.kr
The quick brown fox jumps over the lazy dog.
abbcccdddd
the apple
d
https://local.1234
http://local.12345`

const h = ` the hello  world    ! `
console.log(
  str.match(/[fox]/g) // Both 'f' and 'o' are found
)
console.log(
  str.match(/[0-9]{1,}/g) // All 0-9 characters, one or more, are found
)
console.log(
  str.match(/\w/g) // All English characters are output
)
console.log(
  str.match(/\bf\w{1,}/g) // Breaks at a boundary that is not f ~ 63 characters. ["fox"]
)
console.log(
  h.replace(/\s/g,'') // Can be applied to delete whitespace characters
)

Positive Lookahead, Positive Lookbehind

let str = `
010-1234-5678
s22043@gsm.hs.kr
The quick brown fox jumps over the lazy dog.
abbcccdddd
the apple
d
https://local.1234
http://local.12345`

console.log(
  str.match(/.{1,}(?=@)/g) // Array of one or more characters before '@' ["s22043"]
)
console.log(
  str.match(/(?<=@).{1,}/g) // ["gmail.com"]
)