Why this regex assertion doesn't match?

Question

I\'m trying to just preg content between html tags, I\'m trying this simple assertion pattern and I don\'t understand why it doesn\'t match this string.

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Answer 1

Lookbehinds on debuggex (PCRE, Javascript and Python) cannot be of variable width, meaning that you can use (?<=) which has a fixed width (3 characters) but not something that can vary in length (?<=) (can have 3 characters, or 4, or 5, etc).

The regex simply is not valid but debuggex tells you that there is no match.