Javascript RegExp finding digit grouping

Question

I try to learn Javascript RegExp and got this RegEx somewhere:

"1234567890".match(/(\d{3})+(?!\d)/g) 
["234567890"] //result from log

I could not understand why it is like this. any help explaining this would be appreciated. Thanks!

Answer 1

The regex matches groups of digits with the condition that the group length must be a multiple of 3 and the character trailing the group cannot be a digit.

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This is how it works:

(\d{3})        // match exactly 3 digits
       +       // 1 to n times
        (?!\d) // Negative lookahead - assert that the next character is not a digit

More about lookaheads.

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In your test string, the only possible match is the one returned from the match() call.

See the following snippet for another example:

 var matches = "1234foobar5678901".match(/(\\d{3})+(?!\\d)/g); console.log(matches); // ["234", "678901"]