$str = 'hellow, when i am working, do not coming, this question is really ,let us test some words complex,luck,unlucky,state,unhappy,test,generator,unset';

//把ing結(jié)尾的單詞詞根部分找出來,找到work,com

//前瞻，斷言，零寬度，正預(yù)測

$patn = '/\b\w+(?=ing\b)/';

// preg_match_all($patn,$str,$res);

//把不是ing結(jié)尾的單詞找出來,但單詞長度必須大于3(ing長度)

//前瞻，斷言，零寬度，負預(yù)測

$patn2 = '/\b\w+(?!ing)\w{3}\b/';

// preg_match_all($patn2,$str,$res);

//把un開頭的單詞詞根部分找出來

//回顧，斷言，零寬度，正預(yù)測

$patn1 = '/(?<=\bun)\w+\b/';

// preg_match_all($patn1,$str,$res);

//把不是un開頭的單詞找出來

//斷言，零寬度，負預(yù)測,回顧

$patn3 = '/\b(?!un)\w+\b/';

preg_match_all($patn3,$str,$res);

echo '

';

print_r($res);

echo '';