JavaScript Regex

I denne vejledning lærer du om JavaScript-regulære udtryk (Regex) ved hjælp af eksempler.

I JavaScript, en Reg ular Ex pression (RegEx) er et objekt, som beskriver en sekvens af tegn anvendes til at definere et søgemønster. For eksempel,

 / a… s $ /

Ovenstående kode definerer et RegEx-mønster. Mønsteret er: enhver streng på fem bogstaver, der starter med a og slutter med s .

Et mønster defineret ved hjælp af RegEx kan bruges til at matche mod en streng.

Udtryk Snor Matchet?
/^a… s$/ abs Intet match
alias Match
abyss Match
Alias Intet match
An abacus Intet match

Opret en RegEx

Der er to måder, du kan oprette et regulært udtryk i JavaScript på.

  1. Brug af et regulært udtryk bogstaveligt:
    Det regulære udtryk består af et mønster, der er lukket mellem skråstreg /. For eksempel, 
     cost regularExp = /abc/;
    Her /abc/er et regelmæssigt udtryk.
  2. Brug af RegExp() konstruktorfunktionen :
    Du kan også oprette et regulært udtryk ved at ringe til RegExp()konstruktorfunktionen. For eksempel, 
     const reguarExp = new RegExp('abc');

For eksempel,

 const regex = new RegExp(/^a… s$/); console.log(regex.test('alias')); // true

I eksemplet ovenfor aliasmatcher strengen med RegEx-mønsteret /^a… s$/. Her test()bruges metoden til at kontrollere, om strengen matcher mønsteret.

Der er flere andre metoder til rådighed til brug med JavaScript RegEx. Lad os lære om regulære udtryk selv, før vi udforsker dem.

Hvis du allerede kender det grundlæggende i RegEx, skal du springe til JavaScript RegEx-metoder.

Angiv mønster ved hjælp af RegEx

For at specificere regulære udtryk anvendes metategn. I ovenstående eksempel ( /^a… s$/) ^og $er metategn.

Metakarakterer

Metategn er tegn, der fortolkes på en speciel måde af en RegEx-motor. Her er en liste over metategn:

(). $ * +? () () |

() - Firkantede parenteser

Firkantede parenteser angiver et sæt tegn, du vil matche.

Udtryk Snor Matchet?
(abc) a 1 kamp
ac 2 kampe
Hey Jude Intet match
abc de ca 5 kampe

Her (abc)vil matche, hvis strengen, du prøver at matche, indeholder noget af a, beller c.

Du kan også angive en række tegn ved hjælp af -firkantede parenteser.

(a-e)er det samme som (abcde).

(1-4)er det samme som (1234).

(0-39)er det samme som (01239).

Du kan supplere (invertere) tegnsættet ved hjælp af caret- ^symbolet i starten af ​​en firkantet parentes.

(^abc) means any character except a or b or c.

(^0-9) means any non-digit character.

. - Period

A period matches any single character (except newline '').

Expression String Matched?
… a No match
ac 1 match
acd 1 match
acde 2 matches (contains 4 characters)

^ - Caret

The caret symbol ^ is used to check if a string starts with a certain character.

Expression String Matched?
^a a 1 match
abc 1 match
bac No match
^ab abc 1 match
acb No match (starts with a but not followed by b)

$ - Dollar

The dollar symbol $ is used to check if a string ends with a certain character.

Expression String Matched?
a$ a 1 match
formula 1 match
cab No match

* - Star

The star symbol * matches zero or more occurrences of the pattern left to it.

Expression String Matched?
ma*n mn 1 match
man 1 match
mann 1 match
main No match (a is not followed by n)
woman 1 match

+ - Plus

The plus symbol + matches one or more occurrences of the pattern left to it.

Expression String Matched?
ma+n mn No match (no a character)
man 1 match
mann 1 match
main No match (a is not followed by n)
woman 1 match

? - Question Mark

The question mark symbol ? matches zero or one occurrence of the pattern left to it.

Expression String Matched?
ma?n mn 1 match
man 1 match
mann No match (more than one n character)
main No match (a is not followed by n)
woman 1 match

() - Braces

Consider this code: (n,m). This means at least n, and at most m repetitions of the pattern left to it.

Expression String Matched?
a(2,3) abc dat No match
abc daat 1 match (at daat)
aabc daaat 2 matches (at aabc and daaat)
aabc daaaat 2 matches (at aabc and daaaat)

Let's try one more example. This RegEx (0-9)(2, 4) matches at least 2 digits but not more than 4 digits.

Expression String Matched?
(0-9)(2,4) ab123csde 1 match (match at ab123csde)
12 and 345673 3 matches (12, 3456, 73)
1 and 2 No match

| - Alternation

Vertical bar | is used for alternation (or operator).

Expression String Matched?
a|b cde No match
ade 1 match (match at ade)
acdbea 3 matches (at acdbea)

Here, a|b match any string that contains either a or b

() - Group

Parentheses () is used to group sub-patterns. For example, (a|b|c)xz match any string that matches either a or b or c followed by xz

Expression String Matched?
(a|b|c)xz ab xz No match
abxz 1 match (match at abxz)
axz cabxz 2 matches (at axzbc cabxz)

- Backslash

Backslash is used to escape various characters including all metacharacters. For example,

$a match if a string contains $ followed by a. Here, $ is not interpreted by a RegEx engine in a special way.

If you are unsure if a character has special meaning or not, you can put in front of it. This makes sure the character is not treated in a special way.

Special Sequences

Special sequences make commonly used patterns easier to write. Here's a list of special sequences:

A - Matches if the specified characters are at the start of a string.

Expression String Matched?
Athe the sun Match
In the sun No match

 - Matches if the specified characters are at the beginning or end of a word.

Expression String Matched?
foo football Match
a football Match
foo a football No match
the foo Match
the afoo test Match
the afootest No match

B - Opposite of . Matches if the specified characters are not at the beginning or end of a word.

Expression String Matched?
Bfoo football No match
a football No match
fooB a football Match
the foo No match
the afoo test No match
the afootest Match

d - Matches any decimal digit. Equivalent to (0-9)

Expression String Matched?
d 12abc3 3 matches (at 12abc3)
JavaScript No match

D - Matches any non-decimal digit. Equivalent to (^0-9)

Expression String Matched?
D 1ab34"50 3 matches (at 1ab34"50)
1345 No match

s - Matches where a string contains any whitespace character. Equivalent to ( fv).

Expression String Matched?
s JavaScript RegEx 1 match
JavaScriptRegEx No match

S - Matches where a string contains any non-whitespace character. Equivalent to (fv).

Expression String Matched?
S a b 2 matches (at a b)
No match

w - Matches any alphanumeric character (digits and alphabets). Equivalent to (a-zA-Z0-9_). By the way, underscore _ is also considered an alphanumeric character.

Expression String Matched?
w 12&": ;c 3 matches (at 12&": ;c)
%"> ! No match

W - Matches any non-alphanumeric character. Equivalent to (^a-zA-Z0-9_)

Expression String Matched?
W 1a2%c 1 match (at 1a2%c)
JavaScript No match

 - Matches if the specified characters are at the end of a string.

Expression String Matched?
JavaScript I like JavaScript 1 match
I like JavaScript Programming No match
JavaScript is fun No match

Tip: To build and test regular expressions, you can use RegEx tester tools such as regex101. This tool not only helps you in creating regular expressions, but it also helps you learn it.

Now you understand the basics of RegEx, let's discuss how to use RegEx in your JavaScript code.

JavaScript Regular Expression Methods

As mentioned above, you can either use RegExp() or regular expression literal to create a RegEx in JavaScript.

 const regex1 = /^ab/; const regex2 = new Regexp('/^ab/');

In JavaScript, you can use regular expressions with RegExp() methods: test() and exec().

There are also some string methods that allow you to pass RegEx as its parameter. They are: match(), replace(), search(), and split().

Method Description
exec() Executes a search for a match in a string and returns an array of information. It returns null on a mismatch.
test() Tests for a match in a string and returns true or false.
match() Returns an array containing all the matches. It returns null on a mismatch.
matchAll() Returns an iterator containing all of the matches.
search() Tests for a match in a string and returns the index of the match. It returns -1 if the search fails.
replace() Søger efter et match i en streng og erstatter det matchede substring med et erstatningssubstring.
split() Opdel en streng i en række understrenge.

Eksempel 1: Regulære udtryk

 const string = 'Find me'; const pattern = /me/; // search if the pattern is in string variable const result1 = string.search(pattern); console.log(result1); // 5 // replace the character with another character const string1 = 'Find me'; string1.replace(pattern, 'found you'); // Find found you // splitting strings into array elements const regex1 = /(s,)+/; const result2 = 'Hello world! '.split(regex1); console.log(result2); // ("I", "am", "learning", "JavaScript", "RegEx") // searching the phone number pattern const regex2 = /(d(3))D(d(3))-(d(4))/g; const result3 = regex2.exec('My phone number is: 555 123-4567.'); console.log(result3); // ("555 123-4567", "555", "123", "4567")

Regular Expression Flag

Flag bruges med regulære udtryk, der tillader forskellige muligheder såsom global søgning, store og små bogstaver osv. De kan bruges separat eller sammen.

Flag Beskrivelse
g Udfører en global match (find alle matches)
m Udfører flerlinjekamp
i Udfører store og små bogstaver

Eksempel 2: Modifikator for regulært udtryk

 const string = 'Hello hello hello'; // performing a replacement const result1 = string.replace(/hello/, 'world'); console.log(result1); // Hello world hello // performing global replacement const result2 = string.replace(/hello/g, 'world'); console.log(result2); // Hello world world // performing case-insensitive replacement const result3 = string.replace(/hello/i, 'world'); console.log(result3); // world hello hello // performing global case-insensitive replacement const result4 = string.replace(/hello/gi, 'world'); console.log(result4); // world world world

Eksempel 3: Validering af telefonnummeret

 // program to validate the phone number function validatePhone(num) ( // regex pattern for phone number const re = /^(?((0-9)(3)))?(-. )?((0-9)(3))(-. )?((0-9)(4))$/g; // check if the phone number is valid let result = num.match(re); if (result) ( console.log('The number is valid.'); ) else ( let num = prompt('Enter number in XXX-XXX-XXXX format:'); validatePhone(num); ) ) // take input let number = prompt('Enter a number XXX-XXX-XXXX'); validatePhone(number);

Produktion

 Indtast et nummer XXX-XXX-XXXX: 2343223432 Indtast nummer i XXX-XXX-XXXX-format: 234-322-3432 Nummeret er gyldigt

Eksempel 4: Validering af e-mail-adressen

 // program to validate the email address function validateEmail(email) ( // regex pattern for email const re = /S+@S+.S+/g; // check if the email is valid let result = re.test(email); if (result) ( console.log('The email is valid.'); ) else ( let newEmail = prompt('Enter a valid email:'); validateEmail(newEmail); ) ) // take input let email = prompt('Enter an email: '); validateEmail(email);

Produktion

 Indtast en e-mail: hellohello Indtast en gyldig e-mail: [email protected] E-mailen er gyldig.

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