github.com/go-playground/Validator (go): Package validator implements value validations for structs and individual fields based on tags.  It can also handle Cross-Field and Cross-Struct validation for nested structs and has the ability to dive into arrays and maps of any type.  see more examples https://github.com/go-playground/validator/tree/v9/_examples  Doing things this way is actually the way the standard library does, see the file.Open method here:  The authors return type "error" to avoid the issue discussed in the following, where err is always != nil:  Validator only InvalidValidationError for bad validation input, nil or ValidationErrors as type error; so, in your code all you need to do is check if the error returned is not nil, and if it's not check if error is InvalidValidationError ( if necessary, most of the time it isn't ) type cast it to type ValidationErrors like so err.(validator.ValidationErrors).  Custom Validation functions can be added. Example:  Cross-Field Validation can be done via the following tags:  If, however, some custom cross-field validation is required, it can be done using a custom validation.  Why not just have cross-fields validation tags (i.e. only eqcsfield and not eqfield)?  The reason is efficiency. If you want to check a field within the same struct "eqfield" only has to find the field on the same struct (1 level). But, if we used "eqcsfield" it could be multiple levels down. Example:  Multiple validators on a field will process in the order defined. Example:  Bad Validator definitions are not handled by the library. Example:  Baked In Cross-Field validation only compares fields on the same struct. If Cross-Field + Cross-Struct validation is needed you should implement your own custom validator.  Comma (",") is the default separator of validation tags. If you wish to have a comma included within the parameter (i.e. excludesall=,) you will need to use the UTF-8 hex representation 0x2C, which is replaced in the code as a comma, so the above will become excludesall=0x2C.  Pipe ("|") is the 'or' validation tags deparator. If you wish to have a pipe included within the parameter i.e. excludesall=| you will need to use the UTF-8 hex representation 0x7C, which is replaced in the code as a pipe, so the above will become excludesall=0x7C  Here is a list of the current built in validators:  Tells the validation to skip this struct field; this is particularly handy in ignoring embedded structs from being validated. (Usage: -)  This is the 'or' operator allowing multiple validators to be used and accepted. (Usage: rbg|rgba) <-- this would allow either rgb or rgba colors to be accepted. This can also be combined with 'and' for example ( Usage: omitempty,rgb|rgba)  When a field that is a nested struct is encountered, and contains this flag any validation on the nested struct will be run, but none of the nested struct fields will be validated. This is useful if inside of your program you know the struct will be valid, but need to verify it has been assigned. NOTE: only "required" and "omitempty" can be used on a struct itself.  Same as structonly tag except that any struct level validations will not run.  Allows conditional validation, for example if a field is not set with a value (Determined by the "required" validator) then other validation such as min or max won't run, but if a value is set validation will run.  This tells the validator to dive into a slice, array or map and validate that level of the slice, array or map with the validation tags that follow. Multidimensional nesting is also supported, each level you wish to dive will require another dive tag. dive has some sub-tags, 'keys' & 'endkeys', please see the Keys & EndKeys section just below.  Example #1  Example #2  Keys & EndKeys  These are to be used together directly after the dive tag and tells the validator that anything between 'keys' and 'endkeys' applies to the keys of a map and not the values; think of it like the 'dive' tag, but for map keys instead of values. Multidimensional nesting is also supported, each level you wish to validate will require another 'keys' and 'endkeys' tag. These tags are only valid for maps.  Example #1  Example #2  This validates that the value is not the data types default zero value. For numbers ensures value is not zero. For strings ensures value is not "". For slices, maps, pointers, interfaces, channels and functions ensures the value is not nil.  The field under validation must be present and not empty only if any of the other specified fields are present. For strings ensures value is not "". For slices, maps, pointers, interfaces, channels and functions ensures the value is not nil.  Examples:  The field under validation must be present and not empty only if all of the other specified fields are present. For strings ensures value is not "". For slices, maps, pointers, interfaces, channels and functions ensures the value is not nil.  Example:  The field under validation must be present and not empty only when any of the other specified fields are not present. For strings ensures value is not "". For slices, maps, pointers, interfaces, channels and functions ensures the value is not nil.  Examples:  The field under validation must be present and not empty only when all of the other specified fields are not present. For strings ensures value is not "". For slices, maps, pointers, interfaces, channels and functions ensures the value is not nil.  Example:  This validates that the value is the default value and is almost the opposite of required.  For numbers, length will ensure that the value is equal to the parameter given. For strings, it checks that the string length is exactly that number of characters. For slices, arrays, and maps, validates the number of items.  For numbers, max will ensure that the value is less than or equal to the parameter given. For strings, it checks that the string length is at most that number of characters. For slices, arrays, and maps, validates the number of items.  For numbers, min will ensure that the value is greater or equal to the parameter given. For strings, it checks that the string length is at least that number of characters. For slices, arrays, and maps, validates the number of items.  For strings & numbers, eq will ensure that the value is equal to the parameter given. For slices, arrays, and maps, validates the number of items.  For strings & numbers, ne will ensure that the value is not equal to the parameter given. For slices, arrays, and maps, validates the number of items.  For strings, ints, and uints, oneof will ensure that the value is one of the values in the parameter.  The parameter should be a list of values separated by whitespace.  Values may be strings or numbers.  For numbers, this will ensure that the value is greater than the parameter given. For strings, it checks that the string length is greater than that number of characters. For slices, arrays and maps it validates the number of items.  Example #1  Example #2 (time.Time)  For time.Time ensures the time value is greater than time.Now.UTC().  Same as 'min' above. Kept both to make terminology with 'len' easier.  Example #1  Example #2 (time.Time)  For time.Time ensures the time value is greater than or equal to time.Now.UTC().  For numbers, this will ensure that the value is less than the parameter given. For strings, it checks that the string length is less than that number of characters. For slices, arrays, and maps it validates the number of items.  Example #1  Example #2 (time.Time) For time.Time ensures the time value is less than time.Now.UTC().  Same as 'max' above. Kept both to make terminology with 'len' easier.  Example #1  Example #2 (time.Time)  For time.Time ensures the time value is less than or equal to time.Now.UTC().  This will validate the field value against another fields value either within a struct or passed in field.  Example #1:  Example #2:  Field Equals Another Field (relative)  This does the same as eqfield except that it validates the field provided relative to the top level struct.  This will validate the field value against another fields value either within a struct or passed in field.  Examples:  Field Does Not Equal Another Field (relative)  This does the same as nefield except that it validates the field provided relative to the top level struct.  Only valid for Numbers and time.Time types, this will validate the field value against another fields value either within a struct or passed in field. usage examples are for validation of a Start and End date:  Example #1:  Example #2:  This does the same as gtfield except that it validates the field provided relative to the top level struct.  Only valid for Numbers and time.Time types, this will validate the field value against another fields value either within a struct or passed in field. usage examples are for validation of a Start and End date:  Example #1:  Example #2:  This does the same as gtefield except that it validates the field provided relative to the top level struct.  Only valid for Numbers and time.Time types, this will validate the field value against another fields value either within a struct or passed in field. usage examples are for validation of a Start and End date:  Example #1:  Example #2:  This does the same as ltfield except that it validates the field provided relative to the top level struct.  Only valid for Numbers and time.Time types, this will validate the field value against another fields value either within a struct or passed in field. usage examples are for validation of a Start and End date:  Example #1:  Example #2:  This does the same as ltefield except that it validates the field provided relative to the top level struct.  This does the same as contains except for struct fields. It should only be used with string types. See the behavior of reflect.Value.String() for behavior on other types.  This does the same as excludes except for struct fields. It should only be used with string types. See the behavior of reflect.Value.String() for behavior on other types.  For arrays & slices, unique will ensure that there are no duplicates. For maps, unique will ensure that there are no duplicate values. For slices of struct, unique will ensure that there are no duplicate values in a field of the struct specified via a parameter.  This validates that a string value contains ASCII alpha characters only  This validates that a string value contains ASCII alphanumeric characters only  This validates that a string value contains unicode alpha characters only  This validates that a string value contains unicode alphanumeric characters only  This validates that a string value contains a basic numeric value. basic excludes exponents etc... for integers or float it returns true.  This validates that a string value contains a valid hexadecimal.  This validates that a string value contains a valid hex color including hashtag (#)  This validates that a string value contains a valid rgb color  This validates that a string value contains a valid rgba color  This validates that a string value contains a valid hsl color  This validates that a string value contains a valid hsla color  This validates that a string value contains a valid email This may not conform to all possibilities of any rfc standard, but neither does any email provider accept all possibilities.  This validates that a string value contains a valid file path and that the file exists on the machine. This is done using os.Stat, which is a platform independent function.  This validates that a string value contains a valid url This will accept any url the golang request uri accepts but must contain a schema for example http:// or rtmp://  This validates that a string value contains a valid uri This will accept any uri the golang request uri accepts  This validataes that a string value contains a valid URN according to the RFC 2141 spec.  This validates that a string value contains a valid base64 value. Although an empty string is valid base64 this will report an empty string as an error, if you wish to accept an empty string as valid you can use this with the omitempty tag.  This validates that a string value contains a valid base64 URL safe value according the the RFC4648 spec. Although an empty string is a valid base64 URL safe value, this will report an empty string as an error, if you wish to accept an empty string as valid you can use this with the omitempty tag.  This validates that a string value contains a valid bitcoin address. The format of the string is checked to ensure it matches one of the three formats P2PKH, P2SH and performs checksum validation.  Bitcoin Bech32 Address (segwit)  This validates that a string value contains a valid bitcoin Bech32 address as defined by bip-0173 (https://github.com/bitcoin/bips/blob/master/bip-0173.mediawiki) Special thanks to Pieter Wuille for providng reference implementations.  This validates that a string value contains a valid ethereum address. The format of the string is checked to ensure it matches the standard Ethereum address format Full validation is blocked by https://github.com/golang/crypto/pull/28  This validates that a string value contains the substring value.  This validates that a string value contains any Unicode code points in the substring value.  This validates that a string value contains the supplied rune value.  This validates that a string value does not contain the substring value.  This validates that a string value does not contain any Unicode code points in the substring value.  This validates that a string value does not contain the supplied rune value.  This validates that a string value starts with the supplied string value  This validates that a string value ends with the supplied string value  This validates that a string value contains a valid isbn10 or isbn13 value.  This validates that a string value contains a valid isbn10 value.  This validates that a string value contains a valid isbn13 value.  This validates that a string value contains a valid UUID. Uppercase UUID values will not pass - use `uuid_rfc4122` instead.  This validates that a string value contains a valid version 3 UUID.  Uppercase UUID values will not pass - use `uuid3_rfc4122` instead.  This validates that a string value contains a valid version 4 UUID.  Uppercase UUID values will not pass - use `uuid4_rfc4122` instead.  This validates that a string value contains a valid version 5 UUID.  Uppercase UUID values will not pass - use `uuid5_rfc4122` instead.  This validates that a string value contains only ASCII characters. NOTE: if the string is blank, this validates as true.  This validates that a string value contains only printable ASCII characters. NOTE: if the string is blank, this validates as true.  This validates that a string value contains one or more multibyte characters. NOTE: if the string is blank, this validates as true.  This validates that a string value contains a valid DataURI. NOTE: this will also validate that the data portion is valid base64  This validates that a string value contains a valid latitude.  This validates that a string value contains a valid longitude.  This validates that a string value contains a valid U.S. Social Security Number.  This validates that a string value contains a valid IP Address.  This validates that a string value contains a valid v4 IP Address.  This validates that a string value contains a valid v6 IP Address.  This validates that a string value contains a valid CIDR Address.  This validates that a string value contains a valid v4 CIDR Address.  This validates that a string value contains a valid v6 CIDR Address.  This validates that a string value contains a valid resolvable TCP Address.  This validates that a string value contains a valid resolvable v4 TCP Address.  This validates that a string value contains a valid resolvable v6 TCP Address.  This validates that a string value contains a valid resolvable UDP Address.  This validates that a string value contains a valid resolvable v4 UDP Address.  This validates that a string value contains a valid resolvable v6 UDP Address.  This validates that a string value contains a valid resolvable IP Address.  This validates that a string value contains a valid resolvable v4 IP Address.  This validates that a string value contains a valid resolvable v6 IP Address.  This validates that a string value contains a valid Unix Address.  This validates that a string value contains a valid MAC Address.  Note: See Go's ParseMAC for accepted formats and types:  This validates that a string value is a valid Hostname according to RFC 952 https://tools.ietf.org/html/rfc952  This validates that a string value is a valid Hostname according to RFC 1123 https://tools.ietf.org/html/rfc1123  Full Qualified Domain Name (FQDN)  This validates that a string value contains a valid FQDN.  This validates that a string value appears to be an HTML element tag including those described at https://developer.mozilla.org/en-US/docs/Web/HTML/Element  This validates that a string value is a proper character reference in decimal or hexadecimal format  This validates that a string value is percent-encoded (URL encoded) according to https://tools.ietf.org/html/rfc3986#section-2.1  This validates that a string value contains a valid directory and that it exists on the machine. This is done using os.Stat, which is a platform independent function.  NOTE: When returning an error, the tag returned in "FieldError" will be the alias tag unless the dive tag is part of the alias. Everything after the dive tag is not reported as the alias tag. Also, the "ActualTag" in the before case will be the actual tag within the alias that failed.  Here is a list of the current built in alias tags:  Validator notes:  A collection of validation rules that are frequently needed but are more complex than the ones found in the baked in validators. A non standard validator must be registered manually like you would with your own custom validation functions.  Example of registration and use:  Here is a list of the current non standard validators:  This package panics when bad input is provided, this is by design, bad code like that should not make it to production.
Keywords: error-handling|translation|validation