Validate PCRE named capture group names in template variables

Template variables like {%my-var%} or {%123abc%} produce invalid PCRE
named capture groups, causing silent regex failures and empty results.
Validate each variable name against /^[a-zA-Z_][a-zA-Z0-9_]*$/ in
TemplatesHelper::prepareTemplate() and throw
InvalidTemplateVariableNameException with a clear message on violation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: aymanrb

src/Exception/InvalidTemplateVariableNameException.php

@@ -0,0 +1,9 @@
+<?php
+
+declare(strict_types = 1);
+
+namespace aymanrb\UnstructuredTextParser\Exception;
+
+class InvalidTemplateVariableNameException extends UnstructuredTextParserException
+{
+}

src/Helper/TemplatesHelper.php

@@ -5,6 +5,7 @@
 namespace aymanrb\UnstructuredTextParser\Helper;
 
 use aymanrb\UnstructuredTextParser\Exception\InvalidTemplatesDirectoryException;
+use aymanrb\UnstructuredTextParser\Exception\InvalidTemplateVariableNameException;
 
 class TemplatesHelper
 {
@@ -90,8 +91,26 @@ private function getAllValidTemplates(): array
         return $templates;
     }
 
+    private function validateVariableNames(string $templateText): void
+    {
+        preg_match_all('/\{%([^%:]+)(?::[^%]*)?\%\}/', $templateText, $matches);
+
+        foreach ($matches[1] as $variableName) {
+            if (!preg_match('/^[a-zA-Z_][a-zA-Z0-9_]*$/', $variableName)) {
+                throw new InvalidTemplateVariableNameException(
+                    sprintf(
+                        'Invalid template variable name "%s": must start with a letter or underscore and contain only alphanumeric characters and underscores.',
+                        $variableName
+                    )
+                );
+            }
+        }
+    }
+
     private function prepareTemplate(string $templateText): string
     {
+        $this->validateVariableNames($templateText);
+
         $templateText = preg_quote($templateText, '/');
 
         $templateText = preg_replace(