How to check if String is Number in Swift
Table of Contents
There are many ways to check whether a string is a number in Swift.
Since people refer to a number differently, to get the right solution, we need to know what kind of number we are going to check.
I will classify a number into four categories.
- String with only digits, e.g., 0 to 9
- String with decimal numbers, e.g., ๙
- String represents a number, e.g., ⅚, 7, 𝟠
- String represents a whole number, e.g., 1, 万
String with only digits
If you want to check whether a string contains only 0 to 9, the easiest method is explicitly check for those ten characters (0 to 9).
There might be many ways to do this, but I will show you two ways to do it.
CharacterSet
We declare a character set with all characters that we want to check. Then we check our string against that set.
extension String {
var isNumber: Bool {
let digitsCharacters = CharacterSet(charactersIn: "0123456789")
return CharacterSet(charactersIn: self).isSubset(of: digitsCharacters)
}
}
Regular Expression
You can also use regular expression for this.
We use range(of:options:)
method. This method finds and returns the range of the first occurrence of a given string within the string.
We provide a regular expression pattern as a search string and specify comparing options as .regularExpression
to make the method do a regular expression search.
extension String {
var isNumber: Bool {
return self.range(
of: "^[0-9]*$", // 1
options: .regularExpression) != nil
}
}
1 ^[0-9]*$
is a pattern that matches a string that starts and ends with the numbers 0 to 9.
Both methods yield the same result.
"0123456789".isNumber = true
"๑๒๓๔๕๖๗๘๙".isNumber = false
"⅚".isNumber = false
"㊈".isNumber = false
"𝟠".isNumber = false
"万".isNumber = false
"1️⃣".isNumber = false
"123456.789".isNumber = false
"123,456,789".isNumber = false
"123,456.789".isNumber = false
"SwiftUI 2.0".isNumber = false
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String with Decimal numbers
If you want to check for string contains any characters classified as Decimal Numbers, you can use the built-in character set, CharacterSet.decimalDigits
CharacterSet.decimalDigits
is the set of all characters used to represent the decimal values 0 through 9.
These character sets include a wider range of decimal digits.
For example:
- Decimal digits in other languages, e.g, ๑๒๓ (123 in Thai).
- Decimal digits used in mathematics, i.e., Doublestruck[1] style digits, e.g., 𝟠
I use the same technique in the previous section but with a different character set.
extension String {
var isNumber: Bool {
let characters = CharacterSet.decimalDigits
return CharacterSet(charactersIn: self).isSubset(of: characters)
}
}
Here is the result.
"0123456789".isNumber = true
"๑๒๓๔๕๖๗๘๙".isNumber = true
"⅚".isNumber = false
"㊈".isNumber = false
"𝟠".isNumber = true
"万".isNumber = false
"1️⃣".isNumber = false
"123456.789".isNumber = false
"123,456,789".isNumber = false
"123,456.789".isNumber = false
"SwiftUI 2.0".isNumber = false
String represents a number
There are many strings that can represent a number, for example, "⅚" (Fraction) and "㊈" (Circled Chinese nine).
Swift Character
has a built-in property, isNumber
, that can indicate whether the character represents a number or not.
We use this property along with allSatisfy
to check whether all the characters in a string are a number.
extension String {
var isNumber: Bool {
return self.allSatisfy { character in
character.isNumber
}
}
}
As you can see isNumber
property can detect many Unicode characters representing a number.
"0123456789".isNumber = true
"๑๒๓๔๕๖๗๘๙".isNumber = true
"⅚".isNumber = true
"㊈".isNumber = true
"𝟠".isNumber = true
"万".isNumber = true
"1️⃣".isNumber = true
"123456.789".isNumber = false
"123,456,789".isNumber = false
"123,456.789".isNumber = false
"SwiftUI 2.0".isNumber = false
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String represents a whole number
Character
also has another property, isWholeNumber
, to test for a character that can represent a whole number.
This is similar to String represents a number but will contain fewer characters, e.g., "⅚" (fraction isn't a whole number).
extension String {
var isNumber: Bool {
return self.allSatisfy { character in
character.isWholeNumber
}
}
}
Here is the result.
"0123456789".isNumber = true
"๑๒๓๔๕๖๗๘๙".isNumber = true
"⅚".isNumber = false
"㊈".isNumber = true
"𝟠".isNumber = true
"万".isNumber = true
"1️⃣".isNumber = false
"123456.789".isNumber = false
"123,456,789".isNumber = false
"123,456.789".isNumber = false
"SwiftUI 2.0".isNumber = false
A letter of the alphabet drawn with doubled vertical strokes is called doublestruck, or sometimes blackboard bold (because doublestruck characters provide a means of indicating bold font weight when writing on a blackboard). For example, 𝟘𝟙𝟚𝟛𝟜𝟝𝟞𝟟𝟠𝟡. https://mathworld.wolfram.com/Doublestruck.html ↩︎
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