How to Define Text Field Validation iPhone

make a validation class and pass ur fields and do validation as u like...
for eg.

import <Foundation/Foundation.h>

@interface validations : NSObject {
    UIAlertView *alert<em>email</em>invalid;
    BOOL isValid;

}
@property(nonatomic, retain) UIAlertView *alert<em>email</em>invalid;
-(id)init;
-(BOOL)emailvalidity:(NSString *)emailString;
@end

-(BOOL)emailvalidity:(NSString *)emailString {
    NSString *emailRegex = @"[A-Z0-9a-z._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,4}"; 
    NSPredicate *emailTest = [NSPredicate predicateWithFormat:@"SELF MATCHES %@", emailRegex]; 

return [emailTest evaluateWithObject:emailString];
}

-(void)dealloc {

[alert<em>email</em>invalid release];
    [super dealloc];
} <br />
@end