[Regular Expressions] Match the Same String Twice

Regular Expression Backreferences provide us a method to match a previously captured pattern a second time.

For example we have an string, and we want to any word which appear twice at the same time:

var str = "Time is the the most important thing thing."

var regex = /(the|thing)\s?/g;

Now it catch 'the' & 'thing', but we only want the first appear one.

var regex = /(the|thing)\s?(?=\1)/g;

--------------

Code:

var str = `Time is the the most important thing thing.`;
var regex = /(the|thing)\s?(?=\1)/g;
 
console.log(str.replace(regex, ''));
 
/*
"Time is the most important thing."
*/

And of course, we can do better:

var regex = /(\w+)\s?(?=\1)/g;

----------------------------

Also we can use this tech to extract the html content:

var str = `<b>Bold</b><i>italics</i>`;

So, first we want to match <></>:

So, '\1' means capture the first group. '(?=)' means only the first appear one.

var regex = /<(\w+)><\/\1>/g;

Then we want to add secod catch group of the content:

var regex = /<(\w+)>(.*)<\/\1>/g;

var str = `<b>Bold</b><i>italics</i>`;
var regex = /<(\w+)>(.*)<\/\1>/g;
 
console.log(str.replace(regex, '$2\n'));
 
/*
    "Bold
    italics
"
*/