ReGex MDN

A regular expression is a sequence of characters that specifies a search pattern.

Used to

1. Validate Text.
2. Search through Text.

Create a regular expression

1. Using a regular expression literal, which consists of a pattern enclosed between slashes, as follows:

let re = /ab+c/;

2. Or calling the constructor function of the RegExp object:

let re = new RegExp('ab+c');

Using regular expressions in JavaScript

Regular expressions are used with the RegExp methods test() and exec() and with the String methods match(), replace(), search(), and split().

• exec() Executes a search for a match in a string. It returns an array of information or null on a mismatch.

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

• match() Returns an array containing all of the matches, including capturing groups, or null if no match is found.

• matchAll() Returns an iterator containing all of the matches, including capturing groups.

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

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

• replaceAll() Executes a search for all matches in a string, and replaces the matched substrings with a replacement substring.

• split() Uses a regular expression or a fixed string to break a string into an array of substrings.

Using groups property

When we have a String which we know has a format, we can collect the parts that make up the string to separate it into different variables for our future operations.

const RE_DATE = /([0-9]{4})-([0-9]{2})-([0-9]{2})/;

const matchObj = RE_DATE.exec('1999-12-31');
const year = matchObj[1]; // 1999
const month = matchObj[2]; // 12
const day = matchObj[3]; // 31

Referring to capture groups via numbers has several disadvantages:

• Finding the number of a capture group is a hassle: you have to count parentheses.
• You need to see the regular expression if you want to understand what the groups are for.
• If you change the order of the capture groups, you also have to change the matching code.

However, capture groups are an all-around superior solution. Which is about identifying capture groups via names.

const RE_DATE = /(?<year>[0-9]{4})-(?<month>[0-9]{2})-(?<day>[0-9]{2})/;
const matchObj = RE_DATE.exec('1999-12-31');
const year = matchObj.groups.year; // 1999
const month = matchObj.groups.month; // 12
const day = matchObj.groups.day; // 31

We can combine other functions, for example the replace() and named capture groups

const RE_DATE = /(?<year>[0-9]{4})-(?<month>[0-9]{2})-(?<day>[0-9]{2})/;
console.log('1999-12-31'.replace(
    RE_DATE,
    (g0,y,m,d,offset,input, {year, month, day}) => // (A)
        month+'/'+day+'/'+year));
    // 12/31/1999

We can do the same thing with telephone numbers, dates, emails, sites, etc.