It seems to have a special meaning.
https://github.com/symfony/symfony/blob/master/src/Symfony/Component/ExpressionLanguage/Lexer.php#L72

But I can not see what for.

I noticed the regex. That's how I figured out I could use single quoted strings as a workaround, but I did not find anything beyond the regex, that's why I'm asking.

It seems this is a leftover from the Twig copying.

The original PR #8913 mentions its based on Twig Expression.
And looking the Twig code the regex is identical.
https://github.com/fabpot/Twig/blob/master/lib/Twig/Lexer.php#L44

Now looking at Twig documentation, the # is used for String Interpolation

And in Twig, String Interpolation is handled separately in the lexing process.

Now for the 100 dollar question, should String Interpolation added to the ExpressionLanguage or should the matching of # as special character be removed? 😋

The expression language does not support string interpolation. So, that's a left over that should be removed to fix this issue. Anyone willing to provide a PR?

Sure, I will.
Thanks for the heads-up guys.

Sorry for commenting on this closed issue. I just wanted to ask if the asterisk has a functionality too?
I accidentally added some line breaks to the Expression using annotation:

   /**
     * @Assert\Expression(
     *     "!(this.type in [
     *          constant('App\\Domain\\Model\\Foo\\Foo::bar'),              
     *          constant('App\\Domain\\Model\\Foo\\Foo::baz')
     *     ] and value == null)"
     * )
     */

Throwing:

Unexpected token "operator" of value "*" around position 23.
in vendor/symfony/symfony/src/Symfony/Component/ExpressionLanguage/Parser.php at line 230

Not sure if this woul happen using YAML too:

• Symfony 2.6 - Expression language #12788

Hello.
I think in your case the asterisk is just the multiplication operator.
So indeed, having an operator right after an open square bracket for example is not expected by the parser.