正则匹配之英文单词匹配，\b边界匹配，开始结尾关键字匹配

$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结尾的单词词根部分找出来,找到work,com

//前瞻，断言，零宽度，正预测

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

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

//把不是ing结尾的单词找出来,但单词长度必须大于3(ing长度)

//前瞻，断言，零宽度，负预测

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

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

//把un开头的单词词根部分找出来

//回顾，断言，零宽度，正预测

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

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

//把不是un开头的单词找出来

//断言，零宽度，负预测,回顾

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

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

echo '

';

print_r($res);

echo '';