Non-words (also spelt nonwords) are tokens which do not start with a letter of the alphabet. Examples of non-words are numbers, punctuation but also tokens such as 25-hour, 16-year-old, !mportant, 3D. Tokens such as post-1945, mp3 or CO2 are words because they start with a letter.

The regular expression Sketch Engine users to identify non-words is [^[:alpha:]].* 

Compare word.