CMP1
CMP01
CMP10000
CMP999999

CMPdog1
steve
1CMP1
1
999999999

IDCompany LIKE 'CMP%[0-9]'

IDCompany LIKE 'CMP[0-9]' OR IDCompany LIKE 'CMP[0-9][0-9]' OR .... through to 6 characters

EDIT 1:
This solution ended up working for me:

IDCompany nvarchar(9) NOT NULL CONSTRAINT DEF_Company_IDCompany DEFAULT 'CMP' + CAST((NEXT VALUE FOR dbo.sq_Company) AS nvarchar) CONSTRAINT CHK_Company_IDCompany CHECK (IDCompany LIKE 'CMP%[0-9]' AND ISNUMERIC(SUBSTRING(IDCompany,4,LEN(IDCompany)-3))=1)

where LEFT(IDCompany, 3) = 'CMP'
    and isnumeric(RIGHT(IDCompany, len(IDCompany) - 3)) = 1
    and IDCompany not like '%[.,-]%'

isnumeric(substring(IDCompany,4,len(IDCompany)))=1 and IDCompany not like '%[.,-]%'