You can watch Regular Show streaming on several platforms:

• Hulu
• YouTube TV
• HBO Max (subscription required, may not be available in all countries)
• Amazon Video (buy or download)
• Apple TV (buy or download)
• Fandango At Home (buy or download)

There is no free streaming option currently available for Regular Show. Hulu and YouTube TV seem to be the primary streaming services where the show is available for subscribers.