Checking if string is numeric in dart

This can be simpliefied a bit

void main(args) {
  print(isNumeric(null));
  print(isNumeric(''));
  print(isNumeric('x'));
  print(isNumeric('123x'));
  print(isNumeric('123'));
  print(isNumeric('+123'));
  print(isNumeric('123.456'));
  print(isNumeric('1,234.567'));
  print(isNumeric('1.234,567'));
  print(isNumeric('-123'));
  print(isNumeric('INFINITY'));
  print(isNumeric(double.INFINITY.toString())); // 'Infinity'
  print(isNumeric(double.NAN.toString()));
  print(isNumeric('0x123'));
}

bool isNumeric(String s) {
  if(s == null) {
    return false;
  }
  return double.parse(s, (e) => null) != null;
}

false   // null  
false   // ''  
false   // 'x'  
false   // '123x'  
true    // '123'  
true    // '+123'
true    // '123.456'  
false   // '1,234.567'  
false   // '1.234,567' (would be a valid number in Austria/Germany/...)
true    // '-123'  
false   // 'INFINITY'  
true    // double.INFINITY.toString()
true    // double.NAN.toString()
false   // '0x123'

from double.parse DartDoc

   * Examples of accepted strings:
   *
   *     "3.14"
   *     "  3.14 \xA0"
   *     "0."
   *     ".0"
   *     "-1.e3"
   *     "1234E+7"
   *     "+.12e-9"
   *     "-NaN"

This version accepts also hexadecimal numbers

bool isNumeric(String s) {
  if(s == null) {
    return false;
  }

  // TODO according to DartDoc num.parse() includes both (double.parse and int.parse)
  return double.parse(s, (e) => null) != null || 
      int.parse(s, onError: (e) => null) != null;
}

print(int.parse('0xab'));

true

UPDATE

Since {onError(String source)} is deprecated now you can just use tryParse:

bool isNumeric(String s) {
 if (s == null) {
   return false;
 }
 return double.tryParse(s) != null;
}

for anyone wanting a non native way using regex

RegExp _numeric = RegExp(r'^-?[0-9]+$');

/// check if the string contains only numbers
 bool isNumeric(String str) {
return _numeric.hasMatch(str);
}

Even shorter. Despite the fact it will works with double as well, using num is more accurately.

isNumeric(string) => num.tryParse(string) != null;

num.tryParse inside:

static num tryParse(String input) {
  String source = input.trim();
  return int.tryParse(source) ?? double.tryParse(source);
}

In Dart 2 this method is deprecated

int.parse(s, onError: (e) => null)

instead, use

 bool _isNumeric(String str) {
    if(str == null) {
      return false;
    }
    return double.tryParse(str) != null;
  }