RegExp. I need to advance my expression with leading zeros - regex

Question

I've got my RegExp: '^[0-9]{0,6}$|^[0-9]\d{0,6}[.,]\d{0,2}'.
I need to upgrade condition above to work with an input like '000'. It should format into '0.00'
There is a list of inputs and outputs that i expect to get:
Inputs:
[5555,
55.5,
55.55,
0.50,
555555.55,
000005,
005]
Outputs:
[5555,
55.5,
55.55,
0.50,
555555.55,
0.00005,
0.05]

Answer 1

When working with RegExps it's important to describe, in prose, what it is you want it to match, and then what you want to do with the match.
In this case your RegExp matches a string consisting entirely of 0-6 digits, or a string starting with 1-7 digits, a . or , and then 0-2 digits.
That is: either a string with digits and no ,/., or one with digits and ,/. and as many (up to 2) digits afterwards.
You then ask to convert 000 to 0.00. I'm guessing that you want to normalize numbers to no unnecessary leading zeros, and two decimal digits.
(Now with more examples, I'm guessing that a number with a leading zero and no decimal point should have decimal point added after the first zero).
I agree that using a proper number formatter is probably the way to go, but since we are talking RegExps, here's what I'd do if I had to use RegExps:
Use capture groups, so you can easily see which part matched.
Use a regexp which doesn't capture leading zeros.
Don't try to count in RegExps. Do that in code on the side (if necessary).
Something like:
final _re = RegExp(r"^\d{0,6}$|^(\d{1,7}[,.]\d{0,2})");
String format(String number) {
var match = _re.firstMatch(number);
if (match == null) return null;
var decimals = match[1];
if (decimals != null) return decimals;
var noDecimals = match[0];
if (!noDecimals.startsWith('0')) return noDecimals;
return "0.${noDecimals.substring(1)}";
}
This matches the same strings as your RegExp.