Regex add leading zeros to DD.MM.YYYY string

Question

I was sure something like this had been answered before in some manner however I only managed to find reveresed versions like MM/DD/YYYY etc or special characters like / - and had little luck converting them as my regex knowledge is terrible.

The closest thread I found was this

I am struggling to find a clean approach using regex and javascript to add trailing zeros to the below string format?

dd.mm.yyyy

Eg 1.1.1998 => 01.01.1998 || 10.1.1998 => 10.01.1998

my current approach is along the lines of

const addLeadingZeros = (input: string) => {
   return input.replace(/.([1-9])(?!\d)/g, '.0$1');
}

You have to use regex? Using moment.js is out of the question? — Alexandre Elshobokshy, Aug 12 '21 at 11:28
Yeah I believe so, I am concatenating the date values from seperate string values. — PengProgrammer, Aug 12 '21 at 11:30
@Kordrad so? It's stable and is powerful, it saved my life on more than one occasion :) — Alexandre Elshobokshy, Aug 12 '21 at 11:34
@PengProgrammer If you already have the date parts as separate strings, you can use `("0" + s).slice(-2)` to left-pad each part with a "0" to a final length of 2. — Andrew Morton, Aug 12 '21 at 11:35
@AlexandreElshobokshy that's true but they themself say `Considering using Moment in your project? There may be better modern alternatives.` So if you don't use it in a project yet you should use their suggested alternative. — t.niese, Aug 12 '21 at 11:35
@t.niese this isn't a debate we should have in here, it's a personal choice. It depends on the project you're working on. — Alexandre Elshobokshy, Aug 12 '21 at 11:37
@AndrewMorton this could work, however I was specifically looking for a regex based approach :) Thanks! — PengProgrammer, Aug 12 '21 at 11:43
If str contains only date, [here another simple regex](https://tio.run/##HctBCoAgEEDRvadwEWRQk7oqIjpIBYkMUUiKStDpzdr@xz/VrYL2h4vN3aVEDEYaoqcj3bgACZJLsQ2E/OAxZMgMHp1RGlnLJzaLpl@rBdr9qUteCCirPGh7BWsQjN1Z/r6U0gs) that might suffice. — bobble bubble, Aug 12 '21 at 11:57
Can't you just replace `\b(?=\d\b)` with `0`? For example like [so](https://regex101.com/r/Ax0aoZ/1) — JvdV, Aug 12 '21 at 14:01
@JvdV Yes that will work and I believe is a more elegant solution, dankie. — PengProgrammer, Aug 12 '21 at 14:54

Amessihel · Accepted Answer · 2021-08-12T11:53:03.380

How about:

input.replace(/(?<=^|\.)(\d)(?=\.|$)/g, '0$1');

It means: "replace all digits preceded by the beginning of a line or a dot and followed by a dot or the end of the line". This example works only for day and month parts.

Example (browser console outputs):

"01.01.1998".replace(/(?<=^|\.)(\d)(?=\.|$)/g, '0$1');
← "01.01.1998"
"1.01.1998".replace(/(?<=^|\.)(\d)(?=\.|$)/g, '0$1');
← "01.01.1998"
"1.1.1998".replace(/(?<=^|\.)(\d)(?=\.|$)/g, '0$1');
← "01.01.1998"

(Note: this is not a solution I particularly recommend, I just focused on the asked point.)

Kordrad · Answer 2 · 2021-08-12T11:57:48.587

My solution:

const addLeadingZeros = (input: string) => {
   return input.replaceAll(/\d*(?<![\d])[\d](?=[.])/g, '0$&');
}

The regex: \d*(?<![\d])[\d](?=[.])

You can find the explanation about it here.

const addLeadingZeros = (input) => {
   return input.replaceAll(/\d*(?<![\d])[\d](?=[.])/g, '0$&');
}

console.log(addLeadingZeros('01.02.3456'))
console.log(addLeadingZeros('1.2.2001'))
console.log(addLeadingZeros('01.2.1234'))
console.log(addLeadingZeros('1.12.1998'))
console.log(addLeadingZeros('3.04.2021'))
console.log(addLeadingZeros('5.6.01'))
console.log(addLeadingZeros('5.06.01'))
console.log(addLeadingZeros('05.6.01'))
console.log(addLeadingZeros('7.8.1'))
console.log(addLeadingZeros('7.08.1'))
console.log(addLeadingZeros('07.8.1'))

score 1 · Answer 3 · answered Aug 12 '21 at 15:27

You can also use this regex /^\d\.|(?<=\.)\d\./g it will select the first number and the number after the first dot, so it will only select date and month on your format.

You can use this like the code bellow

let regex = /^\d\.|(?<=\.)\d\./g;
console.log("8.8.2014".replace(regex, 
str => `0${str}`));
console.log("25.06.2014".replace(regex, 
str => `0${str}`));
console.log("9.06.2014".replace(regex, 
str => `0${str}`));
console.log("08.7.2014".replace(regex, 
str => `0${str}`));
console.log("1.1.1998".replace(regex, 
str => `0${str}`));

Now I'm gonna explain it part by part

regex

The regex is broken by a | which tells it to either match expression before it or after it.

The first part is ^\d\., if you know the basics of regex, you'll know that it means see if there is a number right after which there is a dot at the beginning, it selects the date if there is only one digit representing it.

The second part is (?<=\.)\d\., this part first sees if there is a dot before the string that is to be selected, then it sees if there is only one number after it then a dot. It will select the month if there is only one digit representing it.

Here the first part (?<=.) tells java script to seek for dot but not select it, if there is no dot before the number it won't select the string.

Now the last part g makes it select every instance of string which is matched.

So for the first example it selects the strings "8." and "8."

.replace

.replace is a string function taking two argument, first the string/regex to match, in my code it is a regex.

Second is a string or a callback that returns a string, in my code it's a callback.

The callback can get the an argument which is the matched string.

Then I add the "0" before the matched string with a template string.

So the ending output is

Hope my explanation helped. I might be wrong at some places, as I'm still learning and use regex or even the callback of replace not very often. Hope there weren't many mistakes. If I'm wrong somewhere please let me know.

Regex add leading zeros to DD.MM.YYYY string

3 Answers3

regex

.replace