minor #6662 preg_match throw an warning (nicolae-stelian)

This PR was submitted for the 3.1 branch but it was merged into the 2.7 branch instead (closes #6662).

Discussion
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preg_match throw an warning

The line 139 throw an warning: preg_match(): Unknown modifier ']'. Remove from regular expression the "/" character

Commits
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50f0256 preg_match throw an warning

Author: xabbuh

cookbook/security/custom_authentication_provider.rst

@@ -134,7 +134,7 @@ set an authenticated token in the token storage if successful.
         {
             $request = $event->getRequest();
 
-            $wsseRegex = '/UsernameToken Username="([^"]+)", PasswordDigest="([^"]+)", Nonce="([a-zA-Z0-9+/]+={0,2})", Created="([^"]+)"/';
+            $wsseRegex = '/UsernameToken Username="([^"]+)", PasswordDigest="([^"]+)", Nonce="([a-zA-Z0-9+\/]+={0,2})", Created="([^"]+)"/';
             if (!$request->headers->has('x-wsse') || 1 !== preg_match($wsseRegex, $request->headers->get('x-wsse'), $matches)) {
                 return;
             }