This reference explains it quite well:

lookaround actually matches characters, but then gives up the match, returning only the result: match or no match. That is why they are called “assertions”. They do not consume characters in the string, but only assert whether a match is possible or not.

... most regex flavors do not allow you to use just any regex inside a lookbehind, because they cannot apply a regular expression backwards. The regular expression engine needs to be able to figure out how many characters to step back before checking the lookbehind.

Hope that helps.

In reply to Re^5: How to enforce match priority irrespective of string position by eyepopslikeamosquito
in thread How to enforce match priority irrespective of string position by Polyglot

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