Checking if string is numeric in dart - string

Question

I need to find out if a string is numeric in dart. It needs to return true on any valid number type in dart. So far, my solution is
bool isNumeric(String str) {
try{
var value = double.parse(str);
} on FormatException {
return false;
} finally {
return true;
}
}
Is there a native way to do this? If not, is there a better way to do it?

Answer 1

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;
}

Answer 2

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;
}

Answer 3

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);
}

Answer 4

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);
}

Answer 5

if (int.tryParse(value) == null) {
return 'Only Number are allowed';
}

Answer 6

Yes, there is a more "sophisticated" way *rsrs*.
extension Numeric on String {
bool get isNumeric => num.tryParse(this) != null ? true : false;
}
main() {
print("1".isNumeric); // true
print("-1".isNumeric); // true
print("2.5".isNumeric); // true
print("-2.5".isNumeric); // true
print("0x14f".isNumeric); // true
print("2,5".isNumeric); // false
print("2a".isNumeric); // false
}

Answer 7

import 'dart:convert';
void main() {
//------------------------allMatches Example---------------------------------
print('Example 1');
//We want to extract ages from the following string:
String str1 = 'Sara is 26 years old. Maria is 18 while Masood is 8.';
//Declaring a RegExp object with a pattern that matches sequences of digits
RegExp reg1 = new RegExp(r'(\d+)');
//Iterating over the matches returned from allMatches
Iterable allMatches = reg1.allMatches(str1);
var matchCount = 0;
allMatches.forEach((match) {
matchCount += 1;
print('Match ${matchCount}: ' + str1.substring(match.start, match.end));
});
//------------------------firstMatch Example---------------------------------
print('\nExample 2');
//We want to find the first sequence of word characters in the following string:
//Note: A word character is any single letter, number or underscore
String str2 = '#%^!_as22 d3*fg%';
//Declaring a RegExp object with a pattern that matches sequences of word
//characters
RegExp reg2 = new RegExp(r'(\w+)');
//Using the firstMatch function to display the first match found
Match firstMatch = reg2.firstMatch(str2) as Match;
print('First match: ${str2.substring(firstMatch.start, firstMatch.end)}');
//--------------------------hasMatch Example---------------------------------
print('\nExample 3');
//We want to check whether a following strings have white space or not
String str3 = 'Achoo!';
String str4 = 'Bless you.';
//Declaring a RegExp object with a pattern that matches whitespaces
RegExp reg3 = new RegExp(r'(\s)');
//Using the hasMatch method to check strings for whitespaces
print(
'The string "' + str3 + '" contains whitespaces: ${reg3.hasMatch(str3)}');
print(
'The string "' + str4 + '" contains whitespaces: ${reg3.hasMatch(str4)}');
//--------------------------stringMatch Example-------------------------------
print('\nExample 4');
//We want to print the first non-digit sequence in the following strings;
String str5 = '121413dog299toy01food';
String str6 = '00Tom1231frog';
//Declaring a RegExp object with a pattern that matches sequence of non-digit
//characters
RegExp reg4 = new RegExp(r'(\D+)');
//Using the stringMatch method to find the first non-digit match:
String? str5Match = reg4.stringMatch(str5);
String? str6Match = reg4.stringMatch(str6);
print('First match for "' + str5 + '": $str5Match');
print('First match for "' + str6 + '": $str6Match');
//--------------------------matchAsPrefix Example-----------------------------
print('\nExample 5');
//We want to check if the following strings start with the word "Hello" or not:
String str7 = 'Greetings, fellow human!';
String str8 = 'Hello! How are you today?';
//Declaring a RegExp object with a pattern that matches the word "Hello"
RegExp reg5 = new RegExp(r'Hello');
//Using the matchAsPrefix method to match "Hello" to the start of the strings
Match? str7Match = reg5.matchAsPrefix(str7);
Match? str8Match = reg5.matchAsPrefix(str8);
print('"' + str7 + '" starts with hello: ${str7Match != null}');
print('"' + str8 + '" starts with hello: ${str8Match != null}');
}

Answer 8

bool isOnlyNumber(String str) {
try{
var value = int.parse(str);
return true;
} on FormatException {
return false;
}
}

Answer 9

Building on Matso Abgaryan's answer and Günter Zöchbauer's updated answer,
I can get a ',' (comma) on my numeric soft keyboard while developing mobile apps in flutter, and double.tryParse() can return Nan as well as null - so checking vs null is not enough if a user can enter a comma on a soft numeric keyboard. If a string is empty, using double.tryParse() will produce null - so that edge case will be caught using this function;
bool _isNumeric(String str) {
if (str == null) {
return false;
}
return double.tryParse(str) is double;
}
Documentation for tryParse()