newbie at regular expressions needs help

OK, I am reading over some regular expressions tutorials and came
across this:

/th[^s]*./

-- I want to match the words that start
-- with 'th' and end with 's'.

-->this<--
-->thus<--
-->thistle<--
-->this<-- line matches too much

Why does the above not match any word starting with "th", any number
of letters that are NOT "s", and ending in ANY character (the period
symbolizes wildcard)??

This is supposed to be any word that begins in "th", and ends in "s".
Can someone help shed some light on this? Thank you very much,

JJ