I used the idea from Kerry’s answer, but simplified it since I was just looking for something simple for my specific purpose. Here is what I have:
function numberWithCommas(x) { return x.toString().replace(/\B(?=(\d{3})+(?!\d))/g, ","); }
The regex uses 2 lookahead assertions:
- a positive one to look for any point in the string that has a multiple of 3 digits in a row after it,
- a negative assertion to make sure that point only has exactly a multiple of 3 digits. The replacement expression puts a comma there.
For example, if you pass it 123456789.01
, the positive assertion will match every spot to the left of the 7 (since 789
is a multiple of 3 digits, 678
is a multiple of 3 digits, 567
, etc.). The negative assertion checks that the multiple of 3 digits does not have any digits after it. 789
has a period after it so it is exactly a multiple of 3 digits, so a comma goes there. 678
is a multiple of 3 digits but it has a 9
after it, so those 3 digits are part of a group of 4, and a comma does not go there. Similarly for 567
. 456789
is 6 digits, which is a multiple of 3, so a comma goes before that. 345678
is a multiple of 3, but it has a 9
after it, so no comma goes there. And so on. The \B
keeps the regex from putting a comma at the beginning of the string.
@neu-rah mentioned that this function adds commas in undesirable places if there are more than 3 digits after the decimal point. If this is a problem, you can use this function:
function numberWithCommas(x) { var parts = x.toString().split("."); parts[0] = parts[0].replace(/\B(?=(\d{3})+(?!\d))/g, ","); return parts.join("."); }
@t.j.crowder pointed out that now that JavaScript has lookbehind (support info), it can be solved in the regular expression itself:
function numberWithCommas(x) { return x.toString().replace(/\B(?<!\.\d*)(?=(\d{3})+(?!\d))/g, ","); }
(?<!\.\d*)
is a negative lookbehind that says the match can’t be preceded by a .
followed by zero or more digits. The negative lookbehind is faster than the split
and join
solution (comparison), at least in V8.