I want to strip the first 6 character(numbers) from a number, and check that it matches a list of numbers. 
For example, the input value of the number could be: 
`1234567891234567`

Therefore the first 6 extracted would be:
`123456`

I then want to check if `123456` matches: `123456|765321|988721`.

I know that I can extract the first `n` digits with the following: `\d{6}` but then I need to check that these first 6 digits matches one of a list of numbers.