Use sync.OnceValue only on go1.21+

sync.OnceValue was introduced in go1.21. Currently validator's go.mod
states minimum go version to be go1.18.

This commit moves the lazy regex initialization into its own file
lazy.go and provides backwards compatibility using buildtags and the
file lazy_compat.go.

Signed-off-by: Kimmo Lehto <[email protected]>

lazy.go

@@ -0,0 +1,14 @@
+//go:build go1.21
+
+package validator
+
+import (
+	"regexp"
+	"sync"
+)
+
+func lazyRegexCompile(str string) func() *regexp.Regexp {
+	return sync.OnceValue(func() *regexp.Regexp {
+		return regexp.MustCompile(str)
+	})
+}

lazy_compat.go

@@ -0,0 +1,19 @@
+//go:build !go1.21
+
+package validator
+
+import (
+	"regexp"
+	"sync"
+)
+
+func lazyRegexCompile(str string) func() *regexp.Regexp {
+	var regex *regexp.Regexp
+	var once sync.Once
+	return func() *regexp.Regexp {
+		once.Do(func() {
+			regex = regexp.MustCompile(str)
+		})
+		return regex
+	}
+}

regexes.go

@@ -1,10 +1,5 @@
 package validator
 
-import (
-	"regexp"
-	"sync"
-)
-
 const (
 	alphaRegexString                 = "^[a-zA-Z]+$"
 	alphaNumericRegexString          = "^[a-zA-Z0-9]+$"
@@ -79,12 +74,6 @@ const (
 	spicedbTypeRegexString           = "^([a-z][a-z0-9_]{1,61}[a-z0-9]/)?[a-z][a-z0-9_]{1,62}[a-z0-9]$"
 )
 
-func lazyRegexCompile(str string) func() *regexp.Regexp {
-	return sync.OnceValue(func() *regexp.Regexp {
-		return regexp.MustCompile(str)
-	})
-}
-
 var (
 	alphaRegex                 = lazyRegexCompile(alphaRegexString)
 	alphaNumericRegex          = lazyRegexCompile(alphaNumericRegexString)