[wp-trac] [WordPress Trac] #62604: Prevent false-positive in validate_callback() in REST API

[WordPress Trac]

#62604: Prevent false-positive in validate_callback() in REST API
------------------------------+-----------------------------
 Reporter:  Takahashi_Fumiki  |      Owner:  (none)
     Type:  enhancement       |     Status:  new
 Priority:  normal            |  Milestone:  Awaiting Review
Component:  REST API          |    Version:  trunk
 Severity:  normal            |   Keywords:
  Focuses:                    |
------------------------------+-----------------------------
 Related: https://github.com/WP-API/docs/issues/194

 ```php
 'type' => 'string',
 'validate_callback' => function( $should_be_date ) {
      return preg_match( '/^\d{4}-\d{2}-\d{2}$/u', $should_be_date );
 }
 ```

 Above validation expects the parameter should be 'YYYY-MM-DD' format, but
 actually any string as "valid" because `preg_match()` returns 0(falsy
 value) for mismatch.

 So, validation priority should be `is_wp_error()` -> "is true?" -> "else,
 invalid."

 Concerns:

 `strpos()` return 0 for match.

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Ticket URL: <https://core.trac.wordpress.org/ticket/62604>
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