Input mask Textfield with Regex - ios

Question

I try to create an input mask for an UITextField for enter aircraft callsign. The mask should be "F-" and 4 letters. I try it with regular expression but it doesn't work fine. I show you my code. It is impossible to enter any character with it.
if (textField==_tImmat) {
NSString *newString = [textField.text stringByReplacingCharactersInRange:range withString:string];
NSString *expression = #"^(F-)([A-Z]{4})$";
NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:expression options:NSRegularExpressionCaseInsensitive error:nil];
NSUInteger numberOfMatches = [regex numberOfMatchesInString:newString options:0 range:NSMakeRange(0, [newString length])];
if (numberOfMatches == 0){
return NO;
}
Any ideas to create regex which works fine for a string value like this :"F-XXXX" where X is an uppercase letter?
Thanks for your help...

Answer 1

I'm guessing that code is in your shouldChangeCharactersInRange method?
If so, because you are imposing of this specific format, no other string can ever be entered, i.e. unless the value of newString is F-<4 LETTERS>, the method will return NO.
So you can either move your validation to textFieldShouldReturn method:
-(BOOL)textFieldShouldReturn:(UITextField *)textField
{
NSString *expression = #"^(F-)([A-Z]{4})$";
NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:expression options:NSRegularExpressionCaseInsensitive error:nil];
NSUInteger numberOfMatches = [regex numberOfMatchesInString:textField.text options:0 range:NSMakeRange(0, [textField.text length])];
if (numberOfMatches == 0)
{
NSLog(#"wrong format!");
}
else
{
NSLog(#"correct format!");
}
return NO;
}
(by the way, for options you are using NSRegularExpressionCaseInsensitive which means it will allow user to put in both uppercase and lowercase characters and validate them as correct)
If you'd like to validate as the user types and restrict input of illegal characters, I'd change your code to something like this:
NSString *newString = [textField.text stringByReplacingCharactersInRange:range withString:string];
switch (newString.length)
{
case 0:
//allow user empty string
return YES;
case 1:
//check that first character is F
return [newString isEqualToString:#"F"];
case 2:
//check that the second character is -
return [newString isEqualToString:#"F-"];
case 3 ... 6:
//this will check the last four characters based on your regex
{
NSString *expression = #"^(F-)([A-Z]{0,4})$"; //notice the added range
NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:expression options:NSRegularExpressionCaseInsensitive error:nil];
NSUInteger numberOfMatches = [regex numberOfMatchesInString:newString options:0 range:NSMakeRange(0, [newString length])];
if (numberOfMatches == 0)
{
return NO;
}
else
{
return YES;
}
}
default:
//in all other cases validation will fail
return NO;
}

Answer 2

Your validation only accepts the value F-XXXX. Starting with a black text field if the user enters the F, your validation fails because it's not F-XXXX.
You need to let the user enter each character and only fail the validation if the entered text can't possibly ever match the pattern.
There's no reason to use a regular expression for this. Just scan newString. Make sure it starts with an F. If the length is 2 or more, check that the 2nd character is a ```. Then simply make sure the length is less than 6 and the last few characters are the letters A-Z.