Dart how to add commas to a string number

You just forgot get first digits into group. Use this short one:

'12345kWh'.replaceAllMapped(RegExp(r'(\d{1,3})(?=(\d{3})+(?!\d))'), (Match m) => '${m[1]},')

Look at the readable version. In last part of expression I added checking to any not digit char including string end so you can use it with '12 Watt' too.

RegExp reg = RegExp(r'(\d{1,3})(?=(\d{3})+(?!\d))');String Function(Match) mathFunc = (Match match) => '${match[1]},';List<String> tests = [  '0',  '10',  '123',  '1230',  '12300',  '123040',  '12k',  '12 ',];for (String test in tests) {      String result = test.replaceAllMapped(reg, mathFunc);  print('$test -> $result');}

It works perfectly:

0 -> 010 -> 10123 -> 1231230 -> 1,23012300 -> 12,300123040 -> 123,04012k -> 12k12  -> 12 

Try the following regex: (\d{1,3})(?=(\d{3})+$)

This will provide two backreferences, and replacing your number using them like $1,$2, will add commas where they are supposed to be.

import 'package:intl/intl.dart';var f = NumberFormat("###,###.0#", "en_US");print(f.format(int.parse("1000300")));

prints 1,000,300.0check dart's NumberFormat here

The format is specified as a pattern using a subset of the ICU formatting patterns.

• 0 A single digit
• # A single digit, omitted if the value is zero
• . Decimal separator
• - Minus sign
• , Grouping separator
• E Separates mantissa and expontent
• + - Before an exponent, to say it should be prefixed with a plus sign.
• % - In prefix or suffix, multiply by 100 and show as percentage
• ‰ (\u2030) In prefix or suffix, multiply by 1000 and show as per mille
• ¤ (\u00A4) Currency sign, replaced by currency name
• ' Used to quote special characters
• ; Used to separate the positive and negative patterns (if both present)