Validate prepared regex pattern before returning from prepareTemplate()

User-provided regex patterns in template variables (e.g. {%id:[0-9+%})
could silently produce broken PCRE and return empty results with no
indication of the root cause. After building the final pattern, run a
dummy preg_match() and throw InvalidTemplateSyntaxException with the
PCRE error message when the pattern is invalid, enabling fast failure
instead of silent data loss.

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

Author: aymanrb

src/Exception/InvalidTemplateSyntaxException.php

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

src/Helper/TemplatesHelper.php

@@ -5,6 +5,7 @@
 namespace aymanrb\UnstructuredTextParser\Helper;
 
 use aymanrb\UnstructuredTextParser\Exception\InvalidTemplatesDirectoryException;
+use aymanrb\UnstructuredTextParser\Exception\InvalidTemplateSyntaxException;
 use aymanrb\UnstructuredTextParser\Exception\InvalidTemplateVariableNameException;
 
 class TemplatesHelper
@@ -133,10 +134,18 @@ static function (array $matches): string {
             $templateText
         ) ?? $templateText;
 
-        return preg_replace(
+        $preparedPattern = preg_replace(
             self::REGEX_GENERIC_VARIABLE,
             self::REPLACE_GENERIC_VARIABLE,
             $templateText
         ) ?? $templateText;
+
+        if (@preg_match('/' . $preparedPattern . '/s', '') === false) {
+            throw new InvalidTemplateSyntaxException(
+                sprintf('Template produced an invalid regex pattern: %s', preg_last_error_msg())
+            );
+        }
+
+        return $preparedPattern;
     }
 }