An interesting go struct tag expression syntax for field validation, etc.
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Latest commit 9ef372d Feb 3, 2019

go-tagexpr report card GoDoc

An interesting go struct tag expression syntax for field validation, etc.


Validator: A powerful validator that supports struct tag expression


  • Support for a variety of common operator
  • Support for accessing arrays, slices, members of the dictionary
  • Support access to any field in the current structure
  • Support access to nested fields, non-exported fields, etc.
  • Built-in len, sprintf, regexp functions
  • Support single mode and multiple mode to define expression
  • Parameter check subpackage
  • Use offset pointers to directly take values, better performance


package tagexpr_test

import (

	tagexpr ""

func Example() {
	type T struct {
		A  int             `tagexpr:"$<0||$>=100"`
		B  string          `tagexpr:"len($)>1 && regexp('^\\w*$')"`
		C  bool            `tagexpr:"{expr1:(f.g)$>0 && $}{expr2:'C must be true when T.f.g>0'}"`
		d  []string        `tagexpr:"{@:len($)>0 && $[0]=='D'} {msg:sprintf('Invalid d: %v',$)}"`
		e  map[string]int  `tagexpr:"len($)==$['len']"`
		e2 map[string]*int `tagexpr:"len($)==$['len']"`
		f  struct {
			g int `tagexpr:"$"`

	vm := tagexpr.New("tagexpr")
	err := vm.WarmUp(new(T))
	if err != nil {

	t := &T{
		A:  107,
		B:  "abc",
		C:  true,
		d:  []string{"x", "y"},
		e:  map[string]int{"len": 1},
		e2: map[string]*int{"len": new(int)},
		f: struct {
			g int `tagexpr:"$"`

	tagExpr, err := vm.Run(t)
	if err != nil {

	if !tagExpr.Eval("d@").(bool) {

	// Output:
	// true
	// true
	// true
	// C must be true when T.f.g>0
	// Invalid d: [x y]
	// true
	// false
	// 1


Struct tag syntax spec:

type T struct {
	// Single model
    Field1 T1 `tagName:"expression"`
	// Multiple model
    Field2 T2 `tagName:"{exprName:expression} [{exprName2:expression2}]..."`

NOTE: The exprName under the same struct field cannot be the same!

Operator or Operand Explain
true false bool
0 0.0 float64 "0"
'' String
nil nil, undefined
+ Digital addition or string splicing
- Digital subtraction or negative
* Digital multiplication
/ Digital division
% division remainder, as: float64(int64(a)%int64(b))
== eq
!= ne
> gt
>= ge
< lt
<= le
&& Logic and
|| Logic or
() Expression group
(X)$ Struct field value named X
(X.Y)$ Struct field value named X.Y
$ Shorthand for (X)$, omit (X) to indicate current struct field value
(X)$['A'] Map value with key A in the struct field X
(X)$[0] The 0th element of the struct field X(type: map, slice, array)
len((X)$) Built-in function len, the length of struct field X
len() Built-in function len, the length of the current struct field
regexp('^\\w*$', (X)$) Regular match the struct field X, return boolean
regexp('^\\w*$') Regular match the current struct field, return boolean
sprintf('X value: %v', (X)$) fmt.Sprintf, format the value of struct field X

Operator priority(high -> low):

  • () bool string float64 nil !
  • * / %
  • + -
  • < <= > >=
  • == !=
  • &&
  • ||

Field Selector


Expression Selector

If expression is multiple model and exprName is not @:


If expression is single model or exprName is @:



goos: darwin
goarch: amd64
BenchmarkTagExpr-4   	10000000	       195 ns/op	      40 B/op	       4 allocs/op
BenchmarkReflect-4   	10000000	       208 ns/op	      16 B/op	       2 allocs/op

Go to test code