My first thought was, it was wrong, but we only need 4 when using it as regex pattern AFAIK.
I didn’t test this but, 2 looks ok to me. Anyone else who can confirm this?

@nicolas-grekas can you please give your 2 cents here please? Thanks 🙏

This might work :)

So I ran a small test:

expr.yaml

argument: "'App\\Mail\\MailerConfiguration'"

expr.php

<?php

require 'vendor/autoload.php';

use Symfony\Component\ExpressionLanguage\ExpressionLanguage;
use Symfony\Component\Yaml\Yaml;

$lang = new ExpressionLanguage();
$file = Yaml::parseFile('expr.yaml');

var_dump($lang->evaluate($file['argument']));
var_dump($lang->compile($file['argument']))

And this produces:

$ php expr.php
string(26) "AppMailMailerConfiguration"
string(28) ""AppMailMailerConfiguration""

So as far as I can see, we indeed do need the 4 backslashes in the configuration (fyi, if I test without the Yaml and with normal PHP strings, we also need 4 backslashes). So I'm going to close for now, but feel free to comment and share your insights @jbdelhommeau! And as always, thanks for taking the time to fix documentation!

Okay I found the problem ! You're right but so am I.

Show this small test :

expr.yaml

argument_1: '"App\\Mail\\MailerConfiguration"'                                  
argument_2: "'App\\Mail\\MailerConfiguration'"  

<?php
require 'vendor/autoload.php';

use Symfony\Component\ExpressionLanguage\ExpressionLanguage;
use Symfony\Component\Yaml\Yaml;

$lang = new ExpressionLanguage();
$file = Yaml::parseFile('expr.yaml');

var_dump($lang->evaluate($file['argument_1']));
var_dump($lang->compile($file['argument_1']));

var_dump($lang->evaluate($file['argument_2']));
var_dump($lang->compile($file['argument_2']));

Tada and the result are :

$ php expr.php 
string(28) "App\Mail\MailerConfiguration"
string(32) ""App\\Mail\\MailerConfiguration""
string(26) "AppMailMailerConfiguration"
string(28) ""AppMailMailerConfiguration""

In Yaml quote or double quote on declaration of the expression alter expression parsing...

Stranger no ?

In my case, and I don't grant it to you in the documentation, I use a simple quote to declare my expression.

Wouldn't that still deserve a little note in the documentation?

Thanks for coming back to this! This is starting to blow my mind 😄 . First, let's reopen this PR as I think this is worth changing/clarifying.

I extended your script a bit more:

Raw YAML value dump
'App\Mail\MailerConfiguration'
"App\\Mail\\MailerConfiguration"


YAML	"'...'"
AppMailMailerConfiguration
"AppMailMailerConfiguration"

YAML	'"..."'
App\Mail\MailerConfiguration
"App\\Mail\\MailerConfiguration"

PHP	"'...'"
AppMailMailerConfiguration
"AppMailMailerConfiguration"

PHP	'"..."'
AppMailMailerConfiguration
"AppMailMailerConfiguration"

For some reason, double backslashes only work with Yaml '"..."' quotes. @xabbuh as you're the Yaml hero, can you shed some light on this?

I always favor single quotes in Yaml (as yaml double quotes are magic), so should we use double backslashes here with '"..."' and add a line comment about it in the code example? And then, what does this mean for the XML and PHP examples? I think they should remain 4 backslashes?

This looks expected to me. Double-quoted strings in YAML support escape sequences. This requires us to escape backslashes that are not part of escape sequences to be doubled which is not the case in single-quoted strings:

The double-quoted style is specified by surrounding “"” indicators. This is the only style capable of expressing arbitrary strings, by using “\” escape sequences. This comes at the cost of having to escape the “\” and “"” characters.

see https://yaml.org/spec/1.2/spec.html#id2787109

So, what I would do here is to add a comment that explains that you need to escape backslashes when using double-quoted strings.

On a side note, I personally prefer single-quoted strings to not have to think about things like this.

I added a comment in yaml to explain the usage with the double quote.

Does this change seem good for you ?

Thank you for your patience! 🙏