ObjectiveC: NSScanner scanDouble problem

In my iPhone app I am trying to understand if a string is a valid number or not, the code below works most of the time but when I have a value starting with number and ends with text it wrongly returns "true" e.g "34rty"

if([[NSSc开发者_如何学运维anner scannerWithString:value] scanDouble:NULL] ){
     val=[NSNumber numberWithDouble:[value doubleValue]];
}   

what is wrong here?

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scanDouble return via a reference.

NSString *string = @"34rty";
NSScanner *scanner = [NSScanner scannerWithString:string];
double doubleValue;
[scanner scanDouble:&doubleValue];
NSNumber *doubleNumber = [NSNumber numberWithDouble:doubleValue];
NSLog(@"doubleValue: %f", doubleValue);
NSLog(@"doubleNumber: %@", doubleNumber);

NSLog output:

doubleValue: 34.000000
doubleNumber: 34

You will have to scan up to the number if there is preceding text.

As @Benjamin says, a RegEx may be a better option for just checking.

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NSScanner stops at the first matching character. So it finds '3', returns positive as it did scan a double, and then stops running. It doesn't check every character.

A REGEX check is a better choice for this purpose.