javascript regex conditional number formatting

Question

I have a task that has a few rules. Basically, I am separating a number into groups of 3. If the last group only has 1 digit in it, then the last 2 groups have 2 digits instead of 1 group of 3 and 1 group of 1.

For example:

123456789 = 123-456-789
12345678901 = 123-456-789-01
1234567 = 123-45-67

My regex so far has been:

serial.match(/\d{3}|\d+/g)

where serial is a string passed in via function

function format(serial) {
    return serial.replace('/-|/s/g', '').match(/\d{3}|\d+/g).join('-');
}

This gets me the majority of the way there, just doesn't split properly when the last match is {4}.

My progress via fiddle: https://jsfiddle.net/dboots/2mu0yp2b/2/

Is your goal to produce a new string ("formatting") or really to extract the matchs ? — Denys Séguret
– Denys Séguret, Commented Apr 8, 2016 at 11:57
I would be producing a new string via .join() on the match. I'll update question to provide clarity. — Don Boots
– Don Boots, Commented Apr 8, 2016 at 12:03
Why not just check the length of the string, and then slice accordingly? — Ruben Serrate
– Ruben Serrate, Commented Apr 8, 2016 at 12:10

Nina Scholz · Accepted Answer · 2016-04-08 12:26:19Z

7

You can use a regex with lookahead.

The Positive Lookahead looks for the pattern after the equal sign, but does not include it in the match.

x(?=y)

Matches 'x' only if 'x' is followed by 'y'. This is called a lookahead.

For example, /Jack(?=Sprat)/ matches 'Jack' only if it is followed by 'Sprat'. /Jack(?=Sprat|Frost)/ matches 'Jack' only if it is followed by 'Sprat' or 'Frost'. However, neither 'Sprat' nor 'Frost' is part of the match results.

function format(s) {
    return s.toString().replace(/\d{2,3}(?=..)/g, '$&-');
}

document.write(format(123456789) + '<br>');
document.write(format(12345678901) + '<br>');
document.write(format(1234567) + '<br>');

edited Apr 8, 2016 at 12:26

answered Apr 8, 2016 at 12:06

Nina Scholz

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4 Comments

Poul Bak Over a year ago

@Wiktor: That's more letters :-)

Don Boots Over a year ago

Thanks so much for the quick reply. I was looking into lookaheads, but after looking at your answer, I was doing them wrong! This works perfectly! Could you explain how the lookahead is working with ..? I was attempting (?=\d{2}).

Don Boots Over a year ago

Oh perhaps I had a ?=(if) then|else. So this statement says if it's a group of 2-3 numbers it's a match and if the next group is 2 numbers, make that a match as well, therefore ending the regex?

Nina Scholz Over a year ago

kind of, but there is no else.

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