add domain validation, allows most domains.

baked_in.go

@@ -122,6 +122,7 @@ var (
 		"email":                         isEmail,
 		"url":                           isURL,
 		"uri":                           isURI,
+		"domain":                        isDomain,
 		"urn_rfc2141":                   isUrnRFC2141, // RFC 2141
 		"file":                          isFile,
 		"base64":                        isBase64,
@@ -1370,6 +1371,11 @@ func isURL(fl FieldLevel) bool {
 	panic(fmt.Sprintf("Bad field type %T", field.Interface()))
 }
 
+// isDomain is the validation function for validating if the current field's value is a valid domain.
+func isDomain(fl FieldLevel) bool {
+	return domainRegex.MatchString(fl.Field().String())
+}
+
 // isUrnRFC2141 is the validation function for validating if the current field's value is a valid URN as per RFC 2141.
 func isUrnRFC2141(fl FieldLevel) bool {
 	field := fl.Field()

regexes.go

@@ -64,6 +64,7 @@ const (
 	bicRegexString                   = `^[A-Za-z]{6}[A-Za-z0-9]{2}([A-Za-z0-9]{3})?$`
 	semverRegexString                = `^(0|[1-9]\d*)\.(0|[1-9]\d*)\.(0|[1-9]\d*)(?:-((?:0|[1-9]\d*|\d*[a-zA-Z-][0-9a-zA-Z-]*)(?:\.(?:0|[1-9]\d*|\d*[a-zA-Z-][0-9a-zA-Z-]*))*))?(?:\+([0-9a-zA-Z-]+(?:\.[0-9a-zA-Z-]+)*))?$` // numbered capture groups https://semver.org/
 	dnsRegexStringRFC1035Label       = "^[a-z]([-a-z0-9]*[a-z0-9]){0,62}$"
+	domainRegexString                = `^[a-z0-9-\.]{0,61}\.[a-z]{2,}$`
 )
 
 var (
@@ -128,4 +129,5 @@ var (
 	bicRegex                   = regexp.MustCompile(bicRegexString)
 	semverRegex                = regexp.MustCompile(semverRegexString)
 	dnsRegexRFC1035Label       = regexp.MustCompile(dnsRegexStringRFC1035Label)
+	domainRegex                = regexp.MustCompile(domainRegexString)
 )