diff --git a/src/Symfony/Component/Yaml/Parser.php b/src/Symfony/Component/Yaml/Parser.php
index 35e7443d13..885fb9aebe 100644
--- a/src/Symfony/Component/Yaml/Parser.php
+++ b/src/Symfony/Component/Yaml/Parser.php
@@ -161,13 +161,13 @@ class Parser
             Inline::initialize($flags, $this->getRealCurrentLineNb(), $this->filename);
 
             $isRef = $mergeNode = false;
-            if (self::preg_match('#^\-((?P<leadspaces>\s+)(?P<value>.+))?$#u', rtrim($this->currentLine), $values)) {
+            if ('-' === $this->currentLine[0] && self::preg_match('#^\-((?P<leadspaces>\s+)(?P<value>.+))?$#u', rtrim($this->currentLine), $values)) {
                 if ($context && 'mapping' == $context) {
                     throw new ParseException('You cannot define a sequence item when in a mapping', $this->getRealCurrentLineNb() + 1, $this->currentLine, $this->filename);
                 }
                 $context = 'sequence';
 
-                if (isset($values['value']) && self::preg_match('#^&(?P<ref>[^ ]+) *(?P<value>.*)#u', $values['value'], $matches)) {
+                if (isset($values['value']) && '&' === $values['value'][0] && self::preg_match('#^&(?P<ref>[^ ]+) *(?P<value>.*)#u', $values['value'], $matches)) {
                     $isRef = $matches['ref'];
                     $values['value'] = $matches['value'];
                 }
@@ -229,7 +229,7 @@ class Parser
                     $key = (string) $key;
                 }
 
-                if ('<<' === $key && (!isset($values['value']) || !self::preg_match('#^&(?P<ref>[^ ]+)#u', $values['value'], $refMatches))) {
+                if ('<<' === $key && (!isset($values['value']) || '&' !== $values['value'][0] || !self::preg_match('#^&(?P<ref>[^ ]+)#u', $values['value'], $refMatches))) {
                     $mergeNode = true;
                     $allowOverwrite = true;
                     if (isset($values['value'][0]) && '*' === $values['value'][0]) {
@@ -286,7 +286,7 @@ class Parser
                             $data += $parsed; // array union
                         }
                     }
-                } elseif ('<<' !== $key && isset($values['value']) && self::preg_match('#^&(?P<ref>[^ ]++) *+(?P<value>.*)#u', $values['value'], $matches)) {
+                } elseif ('<<' !== $key && isset($values['value']) && '&' === $values['value'][0] && self::preg_match('#^&(?P<ref>[^ ]++) *+(?P<value>.*)#u', $values['value'], $matches)) {
                     $isRef = $matches['ref'];
                     $values['value'] = $matches['value'];
                 }
@@ -671,7 +671,7 @@ class Parser
             return $this->refs[$value];
         }
 
-        if (self::preg_match('/^(?:'.self::TAG_PATTERN.' +)?'.self::BLOCK_SCALAR_HEADER_PATTERN.'$/', $value, $matches)) {
+        if (\in_array($value[0], array('!', '|', '>'), true) && self::preg_match('/^(?:'.self::TAG_PATTERN.' +)?'.self::BLOCK_SCALAR_HEADER_PATTERN.'$/', $value, $matches)) {
             $modifiers = isset($matches['modifiers']) ? $matches['modifiers'] : '';
 
             $data = $this->parseBlockScalar($matches['separator'], preg_replace('#\d+#', '', $modifiers), (int) abs($modifiers));
@@ -773,8 +773,10 @@ class Parser
 
         // determine indentation if not specified
         if (0 === $indentation) {
-            if (self::preg_match('/^ +/', $this->currentLine, $matches)) {
-                $indentation = \strlen($matches[0]);
+            $currentLineLength = \strlen($this->currentLine);
+
+            for ($i = 0; $i < $currentLineLength && ' ' === $this->currentLine[$i]; ++$i) {
+                ++$indentation;
             }
         }
 
@@ -1009,7 +1011,7 @@ class Parser
      */
     private function isBlockScalarHeader(): bool
     {
-        return (bool) self::preg_match('~'.self::BLOCK_SCALAR_HEADER_PATTERN.'$~', $this->currentLine);
+        return '' !== $this->currentLine && (bool) self::preg_match('~'.self::BLOCK_SCALAR_HEADER_PATTERN.'$~', $this->currentLine);
     }
 
     /**