perf: optimize context valid identifier check (#340)

Author: Richtermeister

context.go

@@ -3,11 +3,8 @@ package pongo2
 import (
 	"errors"
 	"fmt"
-	"regexp"
 )
 
-var reIdentifiers = regexp.MustCompile("^[a-zA-Z0-9_]+$")
-
 var autoescape = true
 
 func SetAutoescape(newValue bool) {
@@ -30,7 +27,7 @@ type Context map[string]any
 
 func (c Context) checkForValidIdentifiers() *Error {
 	for k, v := range c {
-		if !reIdentifiers.MatchString(k) {
+		if !isValidIdentifier(k) {
 			return &Error{
 				Sender:    "checkForValidIdentifiers",
 				OrigError: fmt.Errorf("context-key '%s' (value: '%+v') is not a valid identifier", k, v),
@@ -40,6 +37,22 @@ func (c Context) checkForValidIdentifiers() *Error {
 	return nil
 }
 
+func isValidIdentifier(s string) bool {
+	for i := range s {
+		if !isValidIdentifierChar(s[i]) {
+			return false
+		}
+	}
+	return true
+}
+
+func isValidIdentifierChar(c byte) bool {
+	return (c >= 'a' && c <= 'z') ||
+		(c >= 'A' && c <= 'Z') ||
+		(c >= '0' && c <= '9') ||
+		c == '_'
+}
+
 // Update updates this context with the key/value-pairs from another context.
 func (c Context) Update(other Context) Context {
 	for k, v := range other {