RegEx: match all occurrences of X not enclosed by Y - php

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

Answer 2

Description
Assuming your use case is a bit more complex then hello, i said <a>hello</a>; then if you where looking for all the hello in hello, i said <a>after arriving say hello</a> you might want to just capture the good and bad, then use some programming logic to process only the matches you're interested in.
This expression will capture all the <a>...</a> sub strings and all the hello strings. Since the undesirable substring is matched first if the desirable substring appears inside then it won't ever be included in the capture group 1.
<a>.*?<\/a>|\b(hello)\b
Example
Live example: http://ideone.com/jpcqSR
Sample Text
Chello said Hello, i said <a>after arriving say hello</a>
Code
$string = 'Chello said Hello, i said <a>after arriving say hello</a>';
$regex = '/<a>.*?<\/a>|\b(hello)\b/ims';
preg_match_all($regex, $string, $matches);
foreach($matches as $key=>$value){
if ($value[1]) {
echo $key . "=" . $value[0];
}
}
Output
Note the upper case H in hello shows that it was the desired substring.
0=Hello