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) It's not clear if the first letter must be capitalized. This regex assumes not.

This version of the regex matches optional modifiers (ie a hyphen then 2 characters after the code) (https://www.aapc.com/resources/what-are-medical-coding-modifiers). A separate regex has been created on regex101 that ignores modifiers