Error in Laravel installer by composer command

When I try to install Laravel using composer by this command

 composer global require laravel/installer

I got error as:

  https://repo.packagist.org could not be fully loaded (Unable to use a proxy: mal
 formed http_proxy url), package information was loaded from the local cache and
  may be out of date


[Composer\Downloader\TransportException]
 Unable to use a proxy: malformed http_proxy url

How to fix this?

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