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;

 

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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;

 

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

var str = `Bolditalics`;

 

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