JavaScript Regular Expressions

JavaScript Regular Expressions or RegExp.

A regular expression or RegExp is a small programming language that helps to find pattern in data. A RegExp can be used to check if some pattern exists in a different data types.

To use RegExp in JavaScript either we use RegExp constructor or we can declare a RegExp pattern using two forward slashes followed by a flag. We can create a pattern in two ways.

To declare a string we use a single quote, double quote a backtick to declare a regular expression we use two forward slashes and an optional flag. The flag could be g, i, m, s, u or y.

RegExp parameters

A regular expression takes two parameters. One required search pattern and an optional flag.

Pattern

A pattern could be a text or any form of pattern which some sort of similarity. For instance the word spam in an email could be a pattern we are interested to look for in an email or a phone number format number might be our interest to look for.

Flags

Flags are optional parameters in a regular expression which determine the type of searching. Let us see some of the flags:

g: a global flag which means looking for a pattern in whole text
i: case insensitive flag(it searches for both lowercase and uppercase)
m: multi line.

Declaring regular expression without global flag and case insensitive flag.

// without flag
let pattern = 'love'
let regEx = new RegExp(pattern)

Declaring regular expression with global flag and case insensitive flag.

let pattern = 'love'

let flag = 'gi'

let regEx = new RegExp(pattern, flag)

Declaring a regex pattern using RegExp object. Writing the pattern and the flag inside the RegExp constructor

let regEx = new RegExp('love','gi')

Testing for a match

test():Tests for a match in a string. It returns true or false.

const str = 'I love JavaScript'
const pattern = /love/
const result = pattern.test(str)
console.log(result)

Array containing all of the match

match():Returns an array containing all of the matches, including capturing groups, or null if no match is found. If we do not use a global flag, match() returns an array containing the pattern, index, input and group.

const str = 'I love JavaScript'
const pattern = /love/
const result = str.match(pattern)
console.log(result)

output:

["love", index: 2, input: "I love JavaScript", groups: undefined]

const str = 'I love JavaScript'
const pattern = /love/g
const result = str.match(pattern)
console.log(result)

output:
["love"]

search(): Tests for a match in a string. It returns the index of the match, or -1 if the search fails.

const str = 'I love JavaScript'
const pattern = /love/g
const result = str.search(pattern)
console.log(result)

output: 2

Replacing a substring

replace(): Executes a search for a match in a string, and replaces the matched substring with a replacement substring.

const txt = 'Python is the most beautiful language.\
I recommend python for a first programming language'

matchReplaced = txt.replace(/Python|python/, 'JavaScript')
console.log(matchReplaced)

Here we use Global /g

const txt = 'Python is the most beautiful language that a human begin has ever created.\
I recommend python for a first programming language'

matchReplaced = txt.replace(/Python|python/g, 'JavaScript')
console.log(matchReplaced)

Replace

const txt = '%I a%m te%%a%%che%r% a%n%d %% I l%o%ve te%ach%ing.\
T%he%re i%s n%o%th%ing as m%ore r%ewarding a%s e%duc%at%i%ng a%n%d e%m%p%ow%er%ing \
p%e%o%ple.\
I fo%und te%a%ching m%ore i%n%t%er%%es%ting t%h%an any other %jobs.\
D%o%es thi%s m%ot%iv%a%te %y%o%u to b%e a t%e%a%cher.'

matches = txt.replace(/%/g, '')
console.log(matches)

output:

I am teacher and I love teaching.There is nothing as more rewarding as educating and empowering people.I found teaching more interesting than any other jobs.Does this motivate you to be a teacher.

Remeber it:

[]: A set of characters
- [a-c] means, a or b or c
- [a-z] means, any letter a to z
- [A-Z] means, any character A to Z
- [0-3] means, 0 or 1 or 2 or 3
- [0-9] means any number 0 to 9
- [A-Za-z0-9] any character which is a to z, A to Z, 0 to 9
\: uses to escape special characters
- \d mean: match where the string contains digits (numbers from 0-9)
- \D mean: match where the string does not contain digits
. : any character except new line character(\n)
^: starts with
- r'^substring' eg r'^love', a sentence which starts with a word love
- r'[^abc] mean not a, not b, not c.
$: ends with
- r'substring$' eg r'love$', sentence ends with a word love
*: zero or more times
- r'[a]*' means a optional or it can occur many times.
+: one or more times
- r'[a]+' means at least once or more times
?: zero or one times
- r'[a]?' means zero times or once
\b: word bounder, matches with the beginning or ending of a word
{3}: Exactly 3 characters
{3,}: At least 3 characters
{3,8}: 3 to 8 characters
|: Either or
- r'apple|banana' mean either of an apple or a banana
(): Capture and group

Regular Expression cheat sheet

Square Bracket

Let's use square bracket to include lower and upper case.

 
    const pattern = '[Aa]pple' // this square bracket means either A or a
    const txt = `Apple and banana are fruits. 
    An old cliche says an apple a day keeps the  
    doctor way has been replaced by a banana.`
    const matches = txt.match(pattern)

    console.log(matches)
 
    output:  
   [
        'Apple',
        index: 0,
        input: 'Apple and banana are fruits. \n' +
          'An old cliche says an apple a day keeps the  \n' +
          'doctor way has been replaced by a banana.',
        groups: undefined
      ]
 

    const pattern = /[Aa]pple/g  // this square bracket means either A or a
    const txt = `Apple and banana are fruits. 
    An old cliche says an apple a day keeps the  
    doctor way has been replaced by a banana.`
    const matches = txt.match(pattern)

    console.log(matches)
 
    output:        [ 'Apple', 'apple' ]
 

If we want to look for the banana, we write the pattern as follows:

 
    const pattern = /[Aa]pple|[Bb]anana/g  
    // this square bracket mean either A or a
    const txt = `Apple and banana are fruits. 
    An old cliche says an apple a day keeps the  
    doctor way has been replaced by a banana.`
    const matches = txt.match(pattern)

    console.log(matches)
 
    output:     [ 'Apple', 'banana', 'apple', 'banana' ]
 
 

JavaScript Regular Expressions