diff --git a/src/engine.ts b/src/engine.ts
index f69aa6f..b7f6570 100644
--- a/src/engine.ts
+++ b/src/engine.ts
@@ -70,7 +70,7 @@ export class JsonTemplateEngine {
     if (
       /^'.*'$/.test(input) ||
       /^".*"$/.test(input) ||
-      /^[a-zA-Z0-9_]*$/.test(input) ||
+      /^\w+$/.test(input) ||
       input.includes('.')
     ) {
       return `$.${input}`;
diff --git a/src/parser.ts b/src/parser.ts
index bb26166..870f152 100644
--- a/src/parser.ts
+++ b/src/parser.ts
@@ -57,11 +57,11 @@ type PathTypeResult = {
 };
 
 export class JsonTemplateParser {
-  private lexer: JsonTemplateLexer;
+  private readonly lexer: JsonTemplateLexer;
 
-  private options?: EngineOptions;
+  private readonly options?: EngineOptions;
 
-  private pathTypesStack: PathTypeResult[] = [];
+  private readonly pathTypesStack: PathTypeResult[] = [];
 
   // indicates currently how many loops being parsed
   private loopCount = 0;
diff --git a/src/reverse_translator.ts b/src/reverse_translator.ts
index ccaddd8..131a712 100644
--- a/src/reverse_translator.ts
+++ b/src/reverse_translator.ts
@@ -38,7 +38,7 @@ import { BINDINGS_PARAM_KEY, DATA_PARAM_KEY, EMPTY_EXPR, INDENTATION_SPACES } fr
 import { escapeStr } from './utils';
 
 export class JsonTemplateReverseTranslator {
-  private options?: EngineOptions;
+  private readonly options?: EngineOptions;
 
   private level = 0;