what is regex?

Regular expressions or RegEx are patterns used to match specific strings of characters in text. In JavaScript, regex can be used to validate forms, parse strings, replace text, and much more.

So now what is its use in js?

Here I see examples of its use in regex in JavaScript:

• Validate an email:

/^([a-z0-9_.-]+)@([da-z.-]+).([a-z.]{2,6})$/ 

• Parse a string into tokens:

/w+/g 

• Find and replace text:

replace(/(hello)/g, 'hi') 

Regex has many uses, these are just a few examples!

Regex prerequisites?

To learn regex, you only need to know one programming language, such as JavaScript. This article will explain regex and make it easy to understand in JS.

Let’s write our first regex!

In JS, regex can be written in 2 ways:

First method:

const regex = /ab+c/ 

Second method:

const regex = new RegExp("ab+c) 

It doesn’t matter which syntax you use. They will have the same functionality
.

Well, let's check this code together, it doesn't matter which one:

The /ab+c/ pattern means:

• Find a string that starts with a
• Followed by at least one b (the + means 1 or more)
• Ending with c

Here are some valid and invalid examples:

Valid:

• “abbbc”
• “abbbbbbbbc”
• “abc”
• “ac”

Invalid:

• “aabbbc”
• “a.c”
• “abbbccc”
• “aaaabbbcccc”

Using special characters:

You can use characters when looking for one or more special characters or even empty space
For example :

The Dot (.) – Matches Any Character:

The . dot metacharacter will match any single character. For example:

/h.t/ will match hot, hut, h@t, h*t, etc. It will match any single character between hand t.

Valid and invalid example:

/.at/

Valid: cat, bat, sat, mat

Invalid: ct, tat, pat

The Asterisk (*) – Matches 0 or More Characters:

The * asterisk metacharacter will match 0 or more of the previous token. For example:

/a*/ will match a, aa, aaa, aaaaa, etc. It will match any number of a’s.

/h.t*/ will match ht, hattt, h@tttt, h*y, etc. It will match h followed by any single character, followed by 0 or more t ’s.

Valid and invalid example:

/ab*c/

Valid: ac, abc, abbc, abccc

Invalid: aabbc, abxxc

The Plus (+) – Matches 1 or More Characters:

The + plus metacharacter will match 1 or more of the previous token. For example:

/a+/ will match a, aa, aaa, aaaaa, etc. It will match any number of a’s, but must be at least 1.

h.+t will match hat, haaat, h@t, h*ttt, etc. It will match h followed by at least one of any character, followed by t.

Valid and invalid example:

/ab+c/

Valid: abc, abbc, abccc

Invalid: ac, abxxc

The Question Mark (?) – Makes a Token Optional:

The ? question mark metacharacter will make the previous token optional. It will match 0 or 1 times. For example:

/colou?r/ will match color and colour. The u is optional.

h.?t will match ht and hat, but not haat. The middle character is optional (?) so it will match once or not at all.

Valid and invalid example:

/colou?r/

Valid: color, colour

Invalid: coloour, colur

/A?pple/

Valid: Apple, Aple

Invalid: Appple, Apple

Meta Characters in Regex

Meta characters are characters with a special meaning in regex. They help define patterns that are more complex than a basic string of letters.

w – Matches any word character (letters, numbers, underscores)

/w+/
Valid: hello, 123, c_a
Invalid: -ab, *&^

Test text: hello _there123

Matches:

• hello
• _there
• 123

W – Matches any non-word character

/W/
Valid: * , -, &
Invalid: a , 1, _

Test text: hello*_-there&123

Matches:

• *
• _
• -
• &

d – Matches any digit

/d/

Valid: 1, 5, 0
Invalid: a, x, #

Test text: hello123there45
Matches:

• 1
• 2
• 3
• 4
• 5

D – Matches any non-digit

/D/
Valid:a, b, *
Invalid: 1, 3, 7

Test text: hello123there45

Matches:

• h
• e
• l
• l
• o
• t
• h
• e
• r
• e ## s – Matches any whitespace character (spaces, tabs, newlines) /s/ Valid:, ,
Invalid:a, *, 3

Test text:hello there how are you

Matches:

Flags in Regex

Flags are optional parameters that modify the behavior of a regex. They are specified after the final slash. By default, regex has no flags.

g – Global flag

The g flag finds all matches rather than stopping after the first match.
/a/g
Valid: aa – will match both a’s
Invalid: b – no matches

Test text: aaaaa

Matches:

• a
• a
• a
• a
• a

Without the g flag, it would only match the first a.

i – Case insensitive flag

The i flag makes the regex case insensitive.

/a/i
Valid: a, A

Invalid: b

Test text: aAbBcC

Matches:

• a
• A
• b
• B
• c
• C

Without the i flag, it would only match a, b, and c.

m – Multi-line flag

The m flag allows ^ and $ to match newline characters. Without it, they will only match the start and end of the string.

/^a/m

Valid: a
bb – will match a followed by a newline

Invalid: bb
a – will not match

Test text:
a bb a

Matches:

• a

Without the m flag, ^a would not match anything in this test text.

Special Characters in Regex:

Here is an explanation of special characters in regex along with examples:

^ – Matches the beginning of a string

/^a/
Valid: abc, a123

Invalid: bca, 123a

Test text:abc a123 bca 123a

Matches:

• a(only in a123)

$ – Matches the end of a string

/a$/

Valid: abc, 123a

Invalid: bca, a123

Test text: abc a123 bca 123a

Matches:

• a (only in abc)

– Either or

/ab/

Valid:a, b

Invalid: c

Test text: acb

Matches:

• a
• b It will match either a or b.

– Escape character

A backslash escapes special characters to match their literal meaning.
/a/b/

Valid: a/b

Invalid: ab, a

Test text: a/b a ab

Matches:

• a/b The backslash escapes the / so it is matched literally instead of indicating an alternation.

. – Any single character

/a.c/
Valid: abc, a1c, a@c
Invalid: ac, abbc

Test text:abc a1c a@c ac abbc

Matches:

• abc
• a1c
• a@c

It will match any single character between a and c.

I hope this helps explain special characters in regex. Let me know if you have any other questions!

Regex Methods

Regex methods allow you to perform powerful operations on strings using regular expressions. Here are some practical examples of using common regex methods:

.test()

The test() method checks for a match in a string. It is used to validate input.

Example: Validate an email

const regex = /^[a-z0-9._%+-]+@[a-z0-9.-]+.[a-z]{2,4}$/; regex.test('mahdimamashli1383@gmail.com'); // Returns true regex.test('mahdimamashli1383@gmail.co'); // Returns false 

.exec()

The exec() method returns information about the first match. It is used to extract information from a string.

Example: Extract a phone number

const regex = /09(1[0-93[1-92[0129[012])-[0-9]{3}-[0-9]{4}/g; const result = regex.exec('Call me at 0912-345-6789 or 0983-123-456');  // Returns ["0912-345-6789"]  

.match()

The match() method finds all matches in a string and returns them in an array. It is used when you want to extract multiple matches from a string.

Example: Find all zip codes

const regex = /[0-9]{5}/g; 'Zipcodes: 94122, 10010, 90210'.match(regex); // Returns ["94122", "10010", "90210"] 

.search()

The search() method searches for a match and returns the index of the first match. It is used when you want to check if a pattern exists in a string.

Example: Check if a string contains a number

'Hello1'.search(/[0-9]/); // Returns 6  'Hello'.search(/[0-9]/); // Returns -1 

.replace()

The replace() method replaces matches with another string. It is used to perform find and replace operations on strings.

Example: Censor words

'Hello World!'.replace(/World!/, '****'); // Returns 'Hello ****' 

Summary:

We covered a lot of ground in this tutorial! Here are the key takeaways:

• Regex or regular expressions are patterns used to match sequences of characters in strings.
• In JavaScript, regex can be used for validation, parsing strings, replacing text, and more.
• Regex has its own meta language with special characters, quantifiers, anchors, flags, and groups.

The methods used for regex in JS are:

• .test() – Tests for a match
• .exec() – Returns info on the first match
• .match() – Returns all matches
• .search() – Searches for a match and returns the index
• .replace() – Replaces matches with another string

• Regex flags include:

• g – Global search
• i – Case insensitive search
• m – Multi-line search

• Practice and experiment with regex! It is a powerful tool once mastered.

Thank you for reading my comprehensive guide to regular expressions in JavaScript! If you have any feedback, questions, or want to improve the article, please email me.

You can also find more regex examples and projects on my GitHub and contact me on Twitter. I hope this tutorial was helpful! Regex can seem cryptic, but with use it will become second nature.

Examples of practice repositories with regex:
(repository 1) (repository 2) (repository 3)

• MAHDI