EmailStringValidator.h

#import <Foundation/Foundation.h>
#import "TextInputValidator.h"
@interface EmailStringValidator : NSObject <TextInputValidator>

@end

EmailStringValidator.m

#import "EmailStringValidator.h"

@implementation EmailStringValidator

-(BOOL)isValidString:(NSString *)string {
    
    NSString *emailPattern = @"^((([!#$%&'*+\\-/=?^_`{|}~\\w])|([!#$%&'*+\\-/=?^_`{|}~\\w][!#$%&'*+\\-/=?^_`{|}~\\.\\w]{0,}[!#$%&'*+\\-/=?^_`{|}~\\w]))[@]\\w+([-.]\\w+)*\\.\\w+([-.]\\w+)*)$";
    NSError *error = nil;
    NSRegularExpression *regexp = [NSRegularExpression regularExpressionWithPattern:emailPattern options:0 error:&error];
    NSAssert(error != nil, @"Validator Error");
    NSUInteger numberOfMatches = [regexp numberOfMatchesInString:string
                                                        options:0
                                                          range:NSMakeRange(0, [string length])];
    return numberOfMatches >0;
}

@end

TextInputValidator.h

#import <Foundation/Foundation.h>

@protocol TextInputValidator <NSObject>

-(BOOL) isValidString:(NSString *) string;

@end