bug #13334 [Yaml] Fixed #10597: Improved Yaml directive parsing (VictoriaQ)

This PR was submitted for the 2.7 branch but it was merged into the 2.3 branch instead (closes #13334).

Discussion
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[Yaml] Fixed #10597: Improved Yaml directive parsing

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10597
| License       | MIT
| Doc PR        | N/A

There was a problem with the regular expression used to process the YAML directive. The modifier `s` made the regex too greedy, replacing everything but the last line of the YAML. Existing tests using the YAML directive contained only one line, that's why they were passing.

Commits
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95d8ce3 [Yaml] Fixed #10597: Improved Yaml directive parsing

src/Symfony/Component/Yaml/Parser.php

@@ -586,7 +586,7 @@ class Parser
 
         // strip YAML header
         $count = 0;
-        $value = preg_replace('#^\%YAML[: ][\d\.]+.*\n#su', '', $value, -1, $count);
+        $value = preg_replace('#^\%YAML[: ][\d\.]+.*\n#u', '', $value, -1, $count);
         $this->offset += $count;
 
         // remove leading comments

src/Symfony/Component/Yaml/Tests/ParserTest.php

@@ -650,6 +650,17 @@ map_in_map: { foo: { bar: *var } }
 EOF
         ));
     }
+
+    public function testYamlDirective()
+    {
+        $yaml = <<<EOF
+%YAML 1.2
+---
+foo: 1
+bar: 2
+EOF;
+        $this->assertEquals(array('foo' => 1, 'bar' => 2), $this->parser->parse($yaml));
+    }
 }
 
 class B