add phone number validator

Stripe/STPPhoneNumberValidator.h

@@ -0,0 +1,17 @@
+//
+//  STPPhoneNumberValidator.h
+//  Stripe
+//
+//  Created by Jack Flintermann on 10/16/15.
+//  Copyright © 2015 Stripe, Inc. All rights reserved.
+//
+
+#import <Foundation/Foundation.h>
+
+@interface STPPhoneNumberValidator : NSObject
+
++ (BOOL)stringIsValidPhoneNumber:(nonnull NSString *)string;
+
++ (nonnull NSString *)formattedPhoneNumberForString:(nonnull NSString *)string;
+
+@end

Stripe/STPPhoneNumberValidator.m

@@ -0,0 +1,36 @@
+//
+//  STPPhoneNumberValidator.m
+//  Stripe
+//
+//  Created by Jack Flintermann on 10/16/15.
+//  Copyright © 2015 Stripe, Inc. All rights reserved.
+//
+
+#import "STPPhoneNumberValidator.h"
+#import "STPCardValidator.h"
+#import "NSString+Stripe.h"
+
+@implementation STPPhoneNumberValidator
+
++ (BOOL)stringIsValidPhoneNumber:(NSString *)string {
+    return [STPCardValidator sanitizedNumericStringForString:string].length == 10;
+}
+
++ (NSString *)formattedPhoneNumberForString:(NSString *)string {
+    NSString *sanitized = [STPCardValidator sanitizedNumericStringForString:string];
+    if (sanitized.length >= 6) {
+        return [NSString stringWithFormat:@"(%@) %@-%@",
+                [sanitized stp_safeSubstringToIndex:3],
+                [[sanitized stp_safeSubstringToIndex:6] stp_safeSubstringFromIndex:3],
+                [[sanitized stp_safeSubstringToIndex:10] stp_safeSubstringFromIndex:6]
+                ];
+    } else if (sanitized.length >= 3) {
+        return [NSString stringWithFormat:@"(%@) %@",
+                [sanitized stp_safeSubstringToIndex:3],
+                [sanitized stp_safeSubstringFromIndex:3]
+                ];
+    }
+    return sanitized;
+}
+
+@end