Finding out whether a string is numeric or not

I'd suggest using the numberFromString: method from the NSNumberFormatter class, as if the number is not valid, it will return nil; otherwise, it will return you an NSNumber.

NSNumberFormatter *nf = [[[NSNumberFormatter alloc] init] autorelease];
BOOL isDecimal = [nf numberFromString:newString] != nil;

Here's one way that doesn't rely on the limited precision of attempting to parse the string as a number:

NSCharacterSet* notDigits = [[NSCharacterSet decimalDigitCharacterSet] invertedSet];
if ([newString rangeOfCharacterFromSet:notDigits].location == NSNotFound)
{
    // newString consists only of the digits 0 through 9
}

See +[NSCharacterSet decimalDigitCharacterSet] and -[NSString rangeOfCharacterFromSet:].

Validate by regular expression, by pattern "^[0-9]+$", with following method -validateString:withPattern:.

[self validateString:"12345" withPattern:"^[0-9]+$"];

1. If "123.123" is considered
   • With pattern "^[0-9]+(.{1}[0-9]+)?$"
2. If exactly 4 digit numbers, without ".".
   • With pattern "^[0-9]{4}$".
3. If digit numbers without ".", and the length is between 2 ~ 5.
   • With pattern "^[0-9]{2,5}$".
4. With minus sign: "^-?\d+$"

The regular expression can be checked in the online web site.

The helper function is as following.

// Validate the input string with the given pattern and
// return the result as a boolean
- (BOOL)validateString:(NSString *)string withPattern:(NSString *)pattern
{
    NSError *error = nil;
    NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:pattern options:NSRegularExpressionCaseInsensitive error:&error];

    NSAssert(regex, @"Unable to create regular expression");

    NSRange textRange = NSMakeRange(0, string.length);
    NSRange matchRange = [regex rangeOfFirstMatchInString:string options:NSMatchingReportProgress range:textRange];

    BOOL didValidate = NO;

    // Did we find a matching range
    if (matchRange.location != NSNotFound)
        didValidate = YES;

    return didValidate;
}

---

Swift 3 version:

Test in playground.

import UIKit
import Foundation

func validate(_ str: String, pattern: String) -> Bool {
    if let range = str.range(of: pattern, options: .regularExpression) {
        let result = str.substring(with: range)
        print(result)
        return true
    }
    return false
}

let a = validate("123", pattern: "^-?[0-9]+")
print(a)

You could create an NSScanner and simply scan the string:

NSDecimal decimalValue;
NSScanner *sc = [NSScanner scannerWithString:newString];
[sc scanDecimal:&decimalValue];
BOOL isDecimal = [sc isAtEnd];

Check out NSScanner's documentation for more methods to choose from.