not_performant_camelCaseToSnakeCaseString

- (nonnull NSString *)camelCaseToSnakeCaseString
{
     if ([self length] == 0) {
        return @"";
     }

     NSError *error = nil;
     // this regex finds the boundaries between lowercase/uppercase/digits and lets the string be split accordingly.
     NSString *lowercaseToUppercaseBoundary = @"(?<=[a-z])(?=[A-Z])";
     NSString *lowercaseToDigitBoundary = @"(?<=[a-z])(?=[0-9])";
     NSString *digitToUppercaseBoundary = @"(?<=[0-9])(?=[A-Z])";
     NSString *regexStr = [NSString stringWithFormat:@"(%@|%@|%@)", lowercaseToUppercaseBoundary, lowercaseToDigitBoundary, digitToUppercaseBoundary];
     NSRegularExpression *regex = [NSRegularExpression regularExpressionWithPattern:regexStr options:0 error:&error];
 
     NSArray *pieces = [self componentsSeparatedByRegularExpression:regex];
     pieces = [pieces bk_map:^NSString *(NSString *str) {
         return [str lowercaseString];
     }];
     return [pieces componentsJoinedByString:@"_"];
}