static NSPredicate + NSString causing crash.

NSPredicate+CHAdditions.m

@implementation NSPredicate (CHAdditions)
+ (NSPredicate *)sharedValidEmailPredicate {
    static NSPredicate *_sharedPredicate = nil;
    static NSString *_emailRegEx = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        _emailRegEx =
        @"(?:[a-z0-9!#$%\\&'*+/=?\\^_`{|}~-]+(?:\\.[a-z0-9!#$%\\&'*+/=?\\^_`{|}"
        @"~-]+)*|\"(?:[\\x01-\\x08\\x0b\\x0c\\x0e-\\x1f\\x21\\x23-\\x5b\\x5d-\\"
        @"x7f]|\\\\[\\x01-\\x09\\x0b\\x0c\\x0e-\\x7f])*\")@(?:(?:[a-z0-9](?:[a-"
        @"z0-9-]*[a-z0-9])?\\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?|\\[(?:(?:25[0-5"
        @"]|2[0-4][0-9]|[01]?[0-9][0-9]?)\\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-"
        @"9][0-9]?|[a-z0-9-]*[a-z0-9]:(?:[\\x01-\\x08\\x0b\\x0c\\x0e-\\x1f\\x21"
        @"-\\x5a\\x53-\\x7f]|\\\\[\\x01-\\x09\\x0b\\x0c\\x0e-\\x7f])+)\\])";
        _sharedPredicate = [NSPredicate predicateWithFormat:@"SELF MATCHES %@", _emailRegEx];
    });
    
    return _sharedPredicate;
}

///NSString+CHAdditions.m

- (BOOL)isValidEmail {
    return [[NSPredicate sharedValidEmailPredicate] evaluateWithObject:self];
}

/// DRForgotUsernameViewController.m
- (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string {
    NSString *newString = [textField.text stringByReplacingCharactersInRange:range withString:string];
    self.navigationItem.rightBarButtonItem.enabled = (newString.isValidString && newString.isValidEmail);
    
    return YES;
}