JS regular expression to implement the thousands separator

function numberThousands (number, thousandsSeperator = ',') {
    
  return String(number).replace(/(\d)(?=(\d\d\d)+(?!\d))/g, '$1' + thousandsSeperator)
}

explain

First of all, understand ： Positive positive preview （look ahead positive assert） and Positive negative pre check (negative assert)

Positive positive preview （look ahead positive assert）：

In any match pattern Match at the beginning of the string to find the string . This is a non fetch match , in other words , The match does not need to be retrieved for later use . for example ,“Windows(?=95|98|NT|2000)“ Can match "Windows2000" Medium "Windows”, But can't match "Windows3.1" Medium "Windows”. Pre check does not consume characters , in other words , After a match happens , Start the search for the next match immediately after the last match , Instead of starting after the character that contains the prefetch .

Positive negative pre check (negative assert)：

In any mismatch pattern Match at the beginning of the string to find the string . This is a non fetch match , in other words , The match does not need to be retrieved for later use . for example "Windows(?!95|98|NT|2000)“ Can match "Windows3.1" Medium "Windows”, But can't match "Windows2000" Medium "Windows". Pre check does not consume characters , in other words , After a match happens , Start the search for the next match immediately after the last match , Instead of starting after the character that contains the prefetch .

/(\d)(?=(\d\d\d)+(?!\d))/g Regular expression diagram ：

Assuming a string 12345, Regular expressions (\d)(?=(\d\d\d)+) First , matching 1, Find out 1 In the back 234, The forward preview has found , therefore 1 Will be matched to ; Back again , matching 2 , Find out 345, therefore 2 Also matched to , Here's the picture

add (?!\d), Also make sure that the last string of the forward look-up It can't be numbers