8 spaces does not in any way match the language described by that regex. Not a partial match, not a full match. Plugging the first space into a DFA described b

8 spaces does not in any way match the language described by that regex. Not a partial match, not a full match. Plugging the first space into a DFA described by the regex is an immediate failure - there is no exit from the initial state initiated by the space character. It's a non match. Regex engines will say they do match because they are by default checking for for substrings that match the language (such as the initial empty string of each line of grep), not for strings that match the language. *edit: added last paragraph.