RegEx

Regular Expressions is a sequence of characters that defines a search pattern for text

Table of Contents

Literal Characters

• a-z and A-Z

Meta characters

• ( other than literals)

Quantifiers

Position

Flags

Character Classes

character class ⇒ [ xyz ] ⇒ x or y or z

• [.] here the dot is not a meta character it’s a literal dot.

• dash has to be first [-.]

• \. is also literal dot.

• [-] matches -

• [a-z] ⇒ matches from a-z not -

• [^abc] ⇒ which is not a | b | c

• \b[a-zA-Z]{4}\b ⇒ only words with literals and 4 digits

• \b[A-Z][a-z]*\b ⇒ Capital Letter followed by 0 or more small letters

Alternation

• (net | com) ( a | b ) a or b

• The last can be .edu , .com , .in , .dev we wrap in ()

• (\w+\.)?\w+@\w+\.(com|in)

Capturing groups

• Suppose i have names like this

john, doe
durga, prasad
loki, danne
sai, venkat

• I want to change the order like doe john , prasad durga like this

• First we find the pattern and select em and groups which we want to change

• (\w+),\s+(\w+)

• $0 ⇒ gives everything

• $1 ⇒ gives the first group

• $2 ⇒ gives the second goup

• So while replacing we do $2 $1 and replace then we get as desired.
• Suppose I have markdown links and I do want to convert to html links

[google](https://google.com)
[yahoo](https://yahoo.com)

• I am capturing the text inside the [] as group 1
• and the links as group 2
• \[(\w+)\]\((.*)\)
• now for replacing i will use
• <a href="$2" alt="$1"> $1 </a>

<a href="https://google.com" alt="google"> google </a>
<a href="https://yahoo.com" alt="yahoo"> yahoo </a>

Back Referneces

• double word capturing is one of the best examples

This is is is some text 
double double words
 not not sure why why this this is happening

• we need to capture is is , double double …

• (\b\w+)\s\1\b

• Here \1 is the captured group that can be used in the same regEx itself

Test and Match

const finder = new RegExp("xyz")
const regex = /hello/

• RegExp.test(”String”)
• returns true or false of match.

• String.match(”RegExp”)
• gives an array of the matches.

• RegExp.exec("String")
 

• split and replace
• string.split(”regex”)
• returns an array
• s.split(/\s/)
• splits at the space
• split at , or space
• s.split(/[\s,]/)
• s.split(/[\s,]+/) - does split at , or space but not each other
 
• string.replace(”regex”, String)
• IT DOES NOT MODIFY ORIGINAL STRING
• if we wanna replace the original string then
• s = s.rplace(regExp, str)
• s.replace(/\s/g, "-")
• replaces the spaces with -
• s.replace(/\w{6,8}/,"dp")
• any words between 6 and 8 will be replaced by dp
• Suppose i have a stirng and everytime I encounter a vowel i wanna double em

var s = 'egg and bananan'
var r = /([aeiou])/ig

s.replace(r,"$$1")
	

• One more use case is using function
• if the length of the word is >2 make it uppercase else remain it same

function k(match){ if(match.length>2){return match.toUpperCase()}else{return match}}
let s = " the rainbow is of 7 colors namely vibgyor and I am dp"
let r =/\b\w+\b/g
s.replace(r,k)
//the output is
		' THE RAINBOW is of 7 COLORS NAMELY VIBGYOR AND I am dp'
		
		
		
// if we caputre groups we can use the  function (match, gruoup1, group2..){}

References

• Amazing youtube playlist:- https://www.youtube.com/playlist?list=PLRqwX-V7Uu6YEypLuls7iidwHMdCM6o2w

• Amazing website for practicing:- https://regexr.com/