\d

Any digit character; functionally equivalent to [0-9] or [[:digit:]]

\d matches 1, 12, 123, etc., but not 1b7. One or more of any digit characters.

\D

Any non-digit character; functionally equivalent to [^0-9] or [^[:digit:]]

\D matches a, ab, ab&, but not 1. One or more of any character but 0, 1, 2, 3, 4, 5, 6, 7, 8, or 9.

\w

Any "word" character. That is, any alphanumeric character; functionally equivalent to [_A-Za-z0-9] or [_[:alnum:]]

\w matches a, ab, a1, but not !&. One or more upper- or lower-case letters or digits, but not punctuation or other special characters.

\W

Any non-alphanumeric character; functionally equivalent to [^_A-Za-z0-9] or [^_[:alnum:]]

\W matches *, &, but not ace or a1. One or more of any character but upper- or lower-case letters and digits.

\s

Any white space character; space, new line, tab, non-breaking space, etc.; functionally equivalent to [[:space]]

vegetable\s matches "vegetable" followed by any non-white space character. So the phrase "I like vegetables in my soup" would trigger the regex, but "I like a vegetable in my soup" would not.

\S

Any non-white space character; anything other than a space, new line, tab, non-breaking space, etc.; functionally equivalent to [^[:space]]

vegetable\S matches "vegetable" followed by any non-white space character. So the phrase "I like vegetables in my soup" would trigger the regex, but "I like a vegetable in my soup" would not.