You could use the CONTAIN() function to check for specific strings or letters, or you could create a REGEXP to check for any letters: [a-zA-Z]
Hope this helps!
As Wim mentioned, if you're looking for UK postcode patterns, REGEXP_MATCH() is probably the best way to detect them.
Here's a calculation that returns True/False based on whether the postcode is recognized as a UK format:
REGEXP_MATCH([Postal Code], '^[A-Za-z][A-Za-z]?\d\d?[A-Za-z]?\s?\d[A-Za-z][A-Za-z]$' )