Extract first numbers from the String that starts with numbers.

main.dart

void main() {
  /// Temporary string for store number character.
  var tempString = '';

  /// Input string.

  const string = '123sdc123s1d s sd155626sd s ds';

  /// Split the input string in list of characters.

  final chars = string.split('');

  /// Loop through the list and check if character is number or not.
  /// If the character is number then add to the [tempString].
  /// Else stop the loop for checking numbers further.

  for (final char in chars) {
    final data = int.tryParse(char);
    if (data != null) {
      tempString += data.toString();
    } else {
      // If you want all the number from String, uncomment `continue;`
      // continue;
      break;
    }
  }

  /// parse the temp string to int

  final finalInt = int.parse(tempString);

  print(finalInt);
  /// prints => 123
}