The CMS, who maintain these codes, describe the format as:- "HCPCS Level II codes (also known as alpha-numeric codes) consist of a single alphabetical letter followed by 4 numeric digits." (https://www.cms.gov/medicare/coding-billing/healthcare-common-procedure-system) NOTE: This regex will not identify HCPCS Level I codes (AKA CPT codes) Also, some sources say the single letter ranges only from A-V (this regex follows A-Z) Finally, this regex does not match procedure modifiers, which are 2 digit codes that follow a HCPCS code (with hyphen).
(https://www.aapc.com/resources/what-are-medical-coding-modifiers) A separate regex has been created on regex101 supporting these modifiers