regex not working with Wordpress REST API - php

Question

The Wordpress REST API lets you validate a parameter against a regex if you provide one in the API route's schema:
register_rest_route( $this->namespace, "/posts", [
"methods" => "POST",
"permission_callback" => "__return_true",
"callback" => "create_post",
"args" => [
"title" => [
"type" => "string",
"description" => "The posts's title",
"required" => true,
"pattern" => '/^[A-Za-z0-9 ]+$/' // <-- Here's the pattern!
]
]
]);
The pattern I provided should accept all alphanumeric values and single spaces. Indeed, the pattern works if I test it in isolation:
preg_match( '/^[A-Za-z0-9 ]+$/', 'Title'); // Returns 1 as expected
preg_match( '/^[A-Za-z0-9 ]+$/', 'Test Title'); // Returns 1 as expected
preg_match( '/^[A-Za-z0-9 ]+$/', '123'); // Returns 1 as expected
preg_match( '/^[A-Za-z0-9 ]+$/', '&Title&'); // Returns 0 as expected
However, the pattern does not seem to work in the context of the Wordpress API! If I make a request to the /post route passing any of the above test values as the title, I always get the following error:
{
"code": "rest_invalid_param",
"message": "Invalid parameter(s): title",
"data": {
"status": 400,
"params": {
"title": "title does not match pattern /^[A-Za-z0-9 ]+$/."
},
"details": {
"title": {
"code": "rest_invalid_pattern",
"message": "title does not match pattern /^[A-Za-z0-9 ]+$/.",
"data": null
}
}
}
}
I've dug a bit deeper and discovered that internally Wordpress uses this function:
function rest_validate_json_schema_pattern( $pattern, $value ) {
$escaped_pattern = str_replace( '#', '\\#', $pattern );
return 1 === preg_match( '#' . $escaped_pattern . '#u', $value );
}
If I run my test values against this function, preg_match always returns false. Unfortunately I don't know the first thing about regexes so I don't understand what this function is doing. I've read the #u bit should be about unicode characters, but can't figure it out besides that. Can somebody please help?
(The Wordpress docs for using a pattern are here, but I can't work anything out of that either).

Answer 1

Got it. So apparently what's happening is that the rest_validate_json_schema_pattern function is adding the regex's delimiting characters, and since my regex already had delimiting characters, things were not working, so the solution is to provide the regex without any delimiting characters to begin with:
register_rest_route( $this->namespace, "/posts", [
"methods" => "POST",
"permission_callback" => "__return_true",
"callback" => "create_post",
"args" => [
"title" => [
"type" => "string",
"description" => "The posts's title",
"required" => true,
"pattern" => '^[A-Za-z0-9 ]+$' // <-- Regex without delimiting characters
]
]
]);
Confusing if you ask me, but it works!