使用前瞻性否定断言的正则表达式

Question

Using php I would like to detect something in a string depending on whether the part immediately after the matched part does not contain certain characters. I think I may need to use a lookahead negative assertion but it is not working as I would expect so not sure. So for example:

$string = 'test somerandomURL><b>apple</b> peach orange';

I want an expression that will detect everything up to the first > so long as <b>apple</b> does not immediately follow the first >

I tried

if(preg_match("/test(.*)>(?!<b>apple<\/b>)(.*)/i",$string)){
echo 'true since <b>apple</b> does not follow > in string';
}else{
echo 'false since <b>apple</b> follows > in string';
}

When the string contains <b>apple</b> after the > it returns false as I expect and need but when I change the string to have <b>peach</b> after the > instead of <b>apple</b> it still returns false and I need it to return true. Without the bold tags it seems to work as I would expect. Any ideas what I am doing wrong?

Answer 1

You're doing a few things wrong, including not turning on error reporting because you have mismatched () 's and an / that needs escaping.

The first problem is that .* is greedy. So in your string it will match: test somerandomURL><b>apple</b>

Usually the solution to that is to make it lazy with .*? but because of the negative lookahead, laziness will still make it match up to a further > when apple is found.

The solution is a negated character class. [^>]* will match any char except >

/test([^>]*)>(?!<b>apple<\/b>)(.*)/i