no_leading_zeros_text_input_formatter.dart

import 'package:flutter/services.dart';

class NoLeadingZerosTextInputFormatter extends TextInputFormatter {
  @override
  TextEditingValue formatEditUpdate(
    TextEditingValue oldValue,
    TextEditingValue newValue,
  ) {
    final newText = newValue.text;

    final userTypedWhenThereWasZero =
        newText.startsWith("0") && newText.length > 1;

    if (!userTypedWhenThereWasZero) return newValue;

    final regExp = RegExp(r'^0+(?=.)');
    final textWithoutLeadingZeros = newText.replaceAll(regExp, "");

    return TextEditingValue(
      text: textWithoutLeadingZeros,
      selection: newValue.selection.copyWith(
        baseOffset: newText.length - 1,
        extentOffset: newText.length - 1,
      ),
    );
  }
}