RegEx：匹配Y未包含的所有X出现

Question

Is it possible to create a regular expression of a pattern X that is not enclosed by a pattern Y using preg_match in PHP?

for example, consider this string:

hello, i said <a>hello</a>

I want a regex that matches the first hello but not the second... I couldn't think of anything

Answer 1

Use negative look behind lookup:

(?<!<a>)hello