diff --git a/src/Symfony/Component/Dotenv/Dotenv.php b/src/Symfony/Component/Dotenv/Dotenv.php
index bcf93678f6970..685df57d736f8 100644
--- a/src/Symfony/Component/Dotenv/Dotenv.php
+++ b/src/Symfony/Component/Dotenv/Dotenv.php
@@ -25,7 +25,7 @@
  */
 final class Dotenv
 {
-    public const VARNAME_REGEX = '(?i:[A-Z][A-Z0-9_]*+)';
+    public const VARNAME_REGEX = '(?i:_?[A-Z][A-Z0-9_]*+)';
     public const STATE_VARNAME = 0;
     public const STATE_VALUE = 1;
 
@@ -351,8 +351,8 @@ private function lexValue(): string
                 ++$this->cursor;
                 $value = str_replace(['\\"', '\r', '\n'], ['"', "\r", "\n"], $value);
                 $resolvedValue = $value;
-                $resolvedValue = $this->resolveVariables($resolvedValue, $loadedVars);
                 $resolvedValue = $this->resolveCommands($resolvedValue, $loadedVars);
+                $resolvedValue = $this->resolveVariables($resolvedValue, $loadedVars);
                 $resolvedValue = str_replace('\\\\', '\\', $resolvedValue);
                 $v .= $resolvedValue;
             } else {
@@ -374,8 +374,8 @@ private function lexValue(): string
                 }
                 $value = rtrim($value);
                 $resolvedValue = $value;
-                $resolvedValue = $this->resolveVariables($resolvedValue, $loadedVars);
                 $resolvedValue = $this->resolveCommands($resolvedValue, $loadedVars);
+                $resolvedValue = $this->resolveVariables($resolvedValue, $loadedVars);
                 $resolvedValue = str_replace('\\\\', '\\', $resolvedValue);
 
                 if ($resolvedValue === $value && preg_match('/\s+/', $value)) {
diff --git a/src/Symfony/Component/Dotenv/Tests/DotenvTest.php b/src/Symfony/Component/Dotenv/Tests/DotenvTest.php
index 2089e4bca336c..72d0d5630ec9a 100644
--- a/src/Symfony/Component/Dotenv/Tests/DotenvTest.php
+++ b/src/Symfony/Component/Dotenv/Tests/DotenvTest.php
@@ -53,6 +53,7 @@ public static function getEnvDataWithFormatErrors()
             ["FOO=\nBAR=\${FOO:-\'a{a}a}", "Unsupported character \"'\" found in the default value of variable \"\$FOO\". in \".env\" at line 2.\n...\\nBAR=\${FOO:-\'a{a}a}...\n                       ^ line 2 offset 24"],
             ["FOO=\nBAR=\${FOO:-a\$a}", "Unsupported character \"\$\" found in the default value of variable \"\$FOO\". in \".env\" at line 2.\n...FOO=\\nBAR=\${FOO:-a\$a}...\n                       ^ line 2 offset 20"],
             ["FOO=\nBAR=\${FOO:-a\"a}", "Unclosed braces on variable expansion in \".env\" at line 2.\n...FOO=\\nBAR=\${FOO:-a\"a}...\n                    ^ line 2 offset 17"],
+            ['_=FOO', "Invalid character in variable name in \".env\" at line 1.\n..._=FOO...\n  ^ line 1 offset 0"],
         ];
 
         if ('\\' !== \DIRECTORY_SEPARATOR) {
@@ -175,6 +176,10 @@ public static function getEnvData()
             ["FOO=\nBAR=\${FOO:=TEST}", ['FOO' => 'TEST', 'BAR' => 'TEST']],
             ["FOO=\nBAR=\$FOO:=TEST}", ['FOO' => 'TEST', 'BAR' => 'TEST}']],
             ["FOO=foo\nFOOBAR=\${FOO}\${BAR}", ['FOO' => 'foo', 'FOOBAR' => 'foo']],
+
+            // underscores
+            ['_FOO=BAR', ['_FOO' => 'BAR']],
+            ['_FOO_BAR=FOOBAR', ['_FOO_BAR' => 'FOOBAR']],
         ];
 
         if ('\\' !== \DIRECTORY_SEPARATOR) {